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.
What were the errors you ran into when running `rustdoc src/libcollections/lib.rs`? Also, if you want to run the tests, you can just run `rustdoc --test src/libcollections/lib.rs`