added keyremap4macbook

Refs caskroom/homebrew-versions#586

`keyremap4macbook-mountainlion` is a weird one. It was renamed to `karabiner` with the mavericks version. This PR proposes we deleted it from caskroom/versions and add it here as a separate cask with a `depends_on :mac_os` that only goes into `mountain_lion`. I’m not fully convinced, though. As I write this, I wonder if we shouldn’t simply replace it in caskroom/versions.

I wasn’t included into Karabiner because a lot of things differ between them, and we’d essentially be managing two apps in the same cask.