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, 5 insertions, 1 deletions

diff --git a/src/client/context.rs b/src/client/context.rs
index 61a1925..f6ffbef 100644
--- a/src/client/context.rs
+++ b/src/client/context.rs
@@ -78,7 +78,11 @@ impl Context {
     /// impl EventHandler for Handler {
     ///     fn message(&self, ctx: Context, msg: Message) {
     ///         if msg.content == "!changename" {
-    ///             ctx.edit_profile(|e| e.username("Edward Elric"));
+    ///             ctx.edit_profile(|mut e| {
+    ///                 e.username("Edward Elric");
+    ///
+    ///                 e
+    ///             });
     ///         }
     ///     }
     /// }