Make builders mutably borrowed

Change the builders so that they are now mutably borrowed, accepting
`&mut self` instead of `self`. Their methods now return `()` instead of
`Self`.

Upgrade path:

Change code such as the following:

```rust
channel.send_message(|m| m
    .embed(|e| e
        .description("test")
        .title("title")));
```

to the following style:

```rust
channel.send_message(|mut m| {
    m.embed(|mut e| {
        e.description("test");
        e.title("title");

        e
    });

    m
});
```

Closes #159.

1 files changed, 19 insertions, 7 deletions

diff --git a/examples/11_create_message_builder/src/main.rs b/examples/11_create_message_builder/src/main.rs
index 88be5a9..837a4b5 100644
--- a/examples/11_create_message_builder/src/main.rs
+++ b/examples/11_create_message_builder/src/main.rs
@@ -14,13 +14,25 @@ impl EventHandler for Handler {
             // using a builder syntax.
             // This example will create a message that says "Hello, World!", with an embed that has
             // a title, description, and footer.
-            if let Err(why) = msg.channel_id.send_message(|m| m
-                .content("Hello, World!")
-                .embed(|e| e
-                    .title("This is a title")
-                    .description("This is a description")
-                    .footer(|f| f
-                        .text("This is a footer")))) {
+            let msg = msg.channel_id.send_message(|mut m| {
+                m.content("Hello, World!");
+                m.embed(|mut e| {
+                    e.title("This is a title");
+                    e.description("This is a description");
+
+                    e.footer(|mut f| {
+                        f.text("This is a footer");
+
+                        f
+                    });
+
+                    e
+                });
+
+                m
+            });
+
+            if let Err(why) = msg {
                 println!("Error sending message: {:?}", why);
             }
         }