return the $id of the added object when calling add() on angularFireCollection

TL;DR: Bring the semantics of angularFireCollection.add() a bit closer to that of Firebase.push().

This change set allows you to reference the added object by it's uniquely-generated (by Firebase) $id. This can be useful, for example, if one controller adds a new object and then passes control to another controller that wants to pull that object out of firebase using its $id.