Move all grammars that use Git repos to submodules

This makes it so we don't have to redownload all the grammars every time we build the grammars gem. We test that the list of submodules in the repo matches the list in grammars.yml. Eventually we'll be able to test that the list of scopes for each submodule is accurate as well, which was the original motivation for this change.

/cc @bkeepers @arfon @pchaigno 
Maybe? TextMate already hosts most of their stuff on GitHub. The [bundles in the Review directory were explicitly not moved to GitHub](http://lists.macromates.com/textmate-dev/2009-June/013991.html), but maybe they'd reconsider that decision now?

It looks like Fantom is over on BitBucket now: https://bitbucket.org/fantom/fan-1.0 and still using Mercurial. The tmLanguage file doesn't exist in their repo anymore, which is why we're pointing at this old version.

Genshi is developed by Edgewall, the folks behind Trac. Trac supports Git but still feels pretty subversion-y to me. I should dig around a bit and see if they have a Git mirror somewhere.