Add TreeSet example

Someone asked for an example usage of this on IRC, so I tossed together the simplest one. Obviously, this isn't up to snuff, but it's better than nothing.
Whoops:

```
@@ -11,8 +11,9 @@
 //! An ordered map and set implemented as self-balancing binary search
 //! trees. The only requirement for the types is that the key implements
 //! `Ord`.
-
+//!
 //! ## Example
+//!
 //! ```{rust} 
 //! let mut tree_set = TreeSet::new();
 //!    
@@ -23,7 +24,7 @@
 //! for i in tree_set.iter() {
 //!    println!("{}", i) // prints 1, then 2, then 3
 //! }
-//!```
+//! ```

Pushing fix now.
```