Improve PHP snippets.
Rename PHP 'class' snippets.

Contains some more generic PHP snippets. Could probably
do with reorganising the PHP snippets in general.
Use <tab> to expand snippets.
Tidy up tab size and wrap width in filetypes.
Add SuperTab settings.
Lots of tweaking. See below for details:

- Remove YouCompleteMe. Too much hassle with Python versions.
- Require SuperTab, as an alternative to YouCompleteMe
- Set fallback 'omnifunc' for unknown filetypes.
- Recognise alternative extensions for Markdown files.
- Automatically trim trailing whitespace on save (not MD files).
Add mkvirtualenv aliases for Python versions.
Add 'file exists' conditional around statement.