Updated authentication methods to login v2

@katowulf - Please review but **do not merge**. I want to get your thoughts on everything up to this point. There is still more work to be done here: testing (I am going to need to updated `mockfirebase` first...), removing Simple Login code, taking authentication out of this user management file, etc. Please provide any feedback on how things are structured though.
Well it takes the same callback they passed into onAuth(). In our API methods, we require them to pass the same callback to off(), so we could take that approach, then there is no need to remember them.

Alternately, my preference is to have onAuth() return a dispose method (I write all my own classes this way and it's a common angular and backbone approach) that can be invoked to turn off the listener. Since it's scoped, there's nothing to store.

```
onAuth: function(callback) {
   /* stuff... */
   return function() {
      this._ref.off(callback);
   };
}
```