Remove snipmate plugin
Add utilsnips plugin
Make UtilSnips be TextMate-like with tab and shift-tab
Update supertab
Add UltiSnips from honza

See https://github.com/honza/vim-snippets

I'm just copying them here rather than installing the plugin so that I
can hack away.
Dumb down the class and def snippets

I don't like their docstrings, so I've pull them out. It might be nice
to add their docstring generation back if I can implement numpydoc-style
docstrings.