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.
Hmmm, that might be a decent heuristic.

One thing that comes up is http://doc.rust-lang.org/std/collections/hashmap/index.html comes up before http://doc.rust-lang.org/std/collections/hashmap/struct.HashMap.html when you search for 'hashmap', which I've always found a bit irritating. I think I prefer having at least some kind of reasonable overview on the module. Ideally each of these pages lives mostly on its own, with the module having overview and the type having just details.