Add git hooks for rebuilding ctags

This is where I (and many others) stole this:
http://tbaggery.com/2011/08/08/effortless-ctags-with-git.html

Rebuild ctags when committing/rebasing/merging/checkouting etc.
Tags are stored inside .git/tags so that they're already out of the VCS and so
that fugitive.vim is able to take advantage of them.