Enhance DSL for git repos

Since it's currently not trivial to run a private Bower repo, if I want to share internal resources using bower, I have to use Git urls.  Since `bower-rails` makes a nice DSL already, it would be handy if we could specify the version as we do in bundler and have bower-rails sort out the specifics of piecing together the dependency URL.  This'll make a bit less friction for Rails devs editing `Bowerfile`.