Resolves #323, along with a few other minor improvements to Gist support...

I poked around http://localhost:5000/github/ipython/ to try and make sure I didn't break any existing functionality. Let me know if there's any more formal way to check and I'll do more testing.

EDIT: Ah, now I notice travis running...

....

* Makes treelist.html slightly more general to support showing gists as well (removes gistlist.html)
* Adds proper github links to non notebook gist files
* Remove trailing table row for next-prev buttons if there aren't any to show
* Columnizes the user gist view page