Consistency with values emitted by firebase.auth.AuthProvider subclasses

[FrankSalad]

The objects emitted by the credential method of firebase.auth.TwitterAuthProvider, firebase.auth.FacebookAuthProvider, and firebase.auth.GoogleAuthProvider all have different shapes.

This lets me do

var firebasejs = require('firebase');
var credential = firebasejs.auth.FacebookAuthProvider.credential(accessToken);
var auth = firestack.auth().signInWithCredential(credential);