Upgrade UltiSnips to 3.0.

I think this does everything we spoke about:

* Adds vim-snippets as a reference into an optional-bundle area.  I also provided code on how to include them, if you want them to be available.
* Upgraded UltiSnips to 3.0 (including the documentation fix for clearsnippets).
* Changed our snippets to have a default priority of -5 and removed the clearing of snippets everywhere.
* Updated the notes to reflect these changes.

Let me know if there's anything more you'd like me to do here.