Preliminary tests for $firebaseAuth

I took a stab at it. Let me know what you think.
To be consistent with the regular Firebase SDK, we should just do `return deferfed.promise;` here. I know it's a bit odd that we return a `user` object which only contains a single `uid` key, but there is a possibility more things get added to that object in the future. And I'd like to be forwards-compatible with the Firebase SDK on this.