Add a message builder for Context::send_message

Add a message builder to `send_message`. Often only one field - i.e.
`content` - needs to be specified, and the rest can be ignored.

This is a preliminary patch to add rich embed support to messages.

This message builder is used via:

```rust
// assuming in a context with a `channel_id` bound
context.send_message(channel_id, |m| m
    .content("TTS ping!")
    .tts(true));
```

4 files changed, 89 insertions, 15 deletions

diff --git a/src/client/context.rs b/src/client/context.rs
index f4ddd5e..0507bc3 100644
--- a/src/client/context.rs
+++ b/src/client/context.rs
@@ -7,6 +7,7 @@ use super::{STATE, http};
 use super::login_type::LoginType;
 use ::utils::builder::{
     CreateInvite,
+    CreateMessage,
     EditChannel,
     EditGuild,
     EditMember,
@@ -453,7 +454,7 @@ impl Context {
     /// [`User::dm`]: ../model/struct.User.html#method.dm
     pub fn dm<C: Into<ChannelId>>(&self, target_id: C, content: &str)
         -> Result<Message> {
-        self.send_message(target_id.into(), content, "", false)
+        self.send_message(target_id.into(), |m| m.content(content))
     }
 
     pub fn edit_channel<C, F>(&self, channel_id: C, f: F)
@@ -838,9 +839,9 @@ impl Context {
     /// [`ClientError::MessageTooLong`]: enum.ClientError.html#variant.MessageTooLong
     /// [`ClientError::NoChannelId`]: ../enum.ClientError.html#NoChannelId
     /// [`Message`]: ../model/struct.Message.html
-    pub fn say(&self, text: &str) -> Result<Message> {
+    pub fn say(&self, content: &str) -> Result<Message> {
         if let Some(channel_id) = self.channel_id {
-            self.send_message(channel_id, text, "", false)
+            self.send_message(channel_id, |m| m.content(content))
         } else {
             Err(Error::Client(ClientError::NoChannelId))
         }
@@ -896,19 +897,19 @@ impl Context {
     ///
     /// [`Channel`]: ../model/enum.Channel.html
     /// [`ClientError::MessageTooLong`]: enum.ClientError.html#variant.MessageTooLong
-    pub fn send_message<C>(&self, channel_id: C, content: &str, nonce: &str, tts: bool)
-        -> Result<Message> where C: Into<ChannelId> {
-        if let Some(length_over) = Message::overflow_length(content) {
-            return Err(Error::Client(ClientError::MessageTooLong(length_over)));
+    pub fn send_message<C, F>(&self, channel_id: C, f: F) -> Result<Message>
+        where C: Into<ChannelId>, F: FnOnce(CreateMessage) -> CreateMessage {
+        let map = f(CreateMessage::default()).0;
+
+        if let Some(ref content) = map.get(&"content".to_owned()) {
+            if let &&Value::String(ref content) = content {
+                if let Some(length_over) = Message::overflow_length(&content) {
+                    return Err(Error::Client(ClientError::MessageTooLong(length_over)));
+                }
+            }
         }
 
-        let map = ObjectBuilder::new()
-            .insert("content", content)
-            .insert("nonce", nonce)
-            .insert("tts", tts)
-            .build();
-
-        http::send_message(channel_id.into().0, map)
+        http::send_message(channel_id.into().0, Value::Object(map))
     }
 
     pub fn set_game(&self, game: Option<Game>) {
diff --git a/src/client/dispatch.rs b/src/client/dispatch.rs
index 985f03e..6295bc9 100644
--- a/src/client/dispatch.rs
+++ b/src/client/dispatch.rs
@@ -1,5 +1,5 @@
 use std::sync::{Arc, Mutex};
-use std::{mem, thread};
+use std::thread;
 use super::event_store::EventStore;
 use super::login_type::LoginType;
 use super::{STATE, Connection, Context};
diff --git a/src/utils/builder/create_message.rs b/src/utils/builder/create_message.rs
new file mode 100644
index 0000000..dc601a4
--- /dev/null
+++ b/src/utils/builder/create_message.rs
@@ -0,0 +1,71 @@
+use serde_json::Value;
+use std::collections::BTreeMap;
+use std::default::Default;
+
+/// A builder to specify the contents of an [`http::create_message`] request,
+/// primarily meant for use through [`Context::send_message`].
+///
+/// `content` is the only required field.
+///
+/// Note that if you only need to send the content of a message, without
+/// specifying other fields, then [`Context::say`] may be a more preferable
+/// option.
+///
+/// # Examples
+///
+/// Sending a message with a content of `test` and applying text-to-speech:
+///
+/// ```rust,ignore
+/// // assuming you are in a context
+/// context.send_message(message.channel_id, |m|
+///     .content("test")
+///     .tts(true));
+/// ```
+///
+/// [`Context::say`]: ../../client/struct.Context.html#method.say
+/// [`Context::send_message`]: ../../client/struct.Context.html#method.send_message
+/// [`http::create_message`]: ../../client/http/fn.create_message.html
+pub struct CreateMessage(pub BTreeMap<String, Value>);
+
+impl CreateMessage {
+    /// Set the content of the message.
+    ///
+    /// **Note**: Message contents must be under 2000 unicode code points.
+    pub fn content(mut self, content: &str) -> Self {
+        self.0.insert("content".to_owned(), Value::String(content.to_owned()));
+
+        CreateMessage(self.0)
+    }
+
+    /// Set the nonce. This is used for validation of a sent message. You most
+    /// likely don't need to worry about this.
+    ///
+    /// Defaults to empty.
+    pub fn nonce(mut self, nonce: &str) -> Self {
+        self.0.insert("nonce".to_owned(), Value::String(nonce.to_owned()));
+
+        CreateMessage(self.0)
+    }
+
+    /// Set whether the message is text-to-speech.
+    ///
+    /// Think carefully before setting this to `true`.
+    ///
+    /// Defaults to `false`.
+    pub fn tts(mut self, tts: bool) -> Self {
+        self.0.insert("tts".to_owned(), Value::Bool(tts));
+
+        CreateMessage(self.0)
+    }
+}
+
+impl Default for CreateMessage {
+    /// Creates a map for sending a [`Message`], setting `tts` to `false` by
+    /// default.
+    fn default() -> CreateMessage {
+        let mut map = BTreeMap::default();
+        map.insert("tts".to_owned(), Value::Bool(false));
+
+        CreateMessage(map)
+    }
+}
diff --git a/src/utils/builder/mod.rs b/src/utils/builder/mod.rs
index cd830fa..91ad940 100644
--- a/src/utils/builder/mod.rs
+++ b/src/utils/builder/mod.rs
@@ -7,6 +7,7 @@
 
 mod create_embed;
 mod create_invite;
+mod create_message;
 mod edit_channel;
 mod edit_guild;
 mod edit_member;
@@ -17,6 +18,7 @@ mod get_messages;
 
 pub use self::create_embed::CreateEmbed;
 pub use self::create_invite::CreateInvite;
+pub use self::create_message::CreateMessage;
 pub use self::edit_channel::EditChannel;
 pub use self::edit_guild::EditGuild;
 pub use self::edit_member::EditMember;