Fix description comment in ftdetect/gitignore.vim
Use single quotes in syntax/gitignore.vim

For two reasons:
 - to be consistent
 - because escape sequences are not used
Set 'commentstring' option for gitignore filetype

This should work for all commenting plugins, so probably code specific
to tCommenter in the plugins/gitignore.vim script can be removed.
Fix symbolic link in one of Snipmate snippets

Use to point to Clojure.snippet used to point to "Leiningen.gitignore"
instead of "Leiningen.snippet".