Minor fixes to $firebaseAuth

@katowulf - A few small fixes for `$firebaseAuth`
Callbacks aren't terribly convenient in all architectures. An event broadcasting strategy can be pretty handy for things like this. It's not impossible or particularly hard to add a .run() method that implements this, if one is versed in Angular and AngularFire, but it's worth pondering as part of the API as lots of people utilize $emit and $broadcast as a basic pattern (particularly when talking to directives)