Store tags in .git to use ag tab completion

Modifying the git hook `ctags` to store tags for any folder under `.git/tags`
rather than in the folder root. This moves the tags to a cleaner place, makes
them less likely to get picked up by other shell commands or git (although it is
in gitignore), and helps take advantage of thoughtbot's upstream ag tab
completion:

http://robots.thoughtbot.com/silver-searcher-tab-completion-with-exuberant-ctags