Getting user data adds another roundtrip to the server, it seems like preliminary pessimization to me.

 1. Change the first arg of LoginCallback.onSuccess to AccessToken.
 
 2. Optionally implement a helper class inheriting from LoginCallback that calls getUser:

  ```java
  abstract class GetLoggedInUserCallback:
          extends LoginCallback implements ModelRepository.FindCallback<User> {
      public abstract void onSuccess(User user);

      @Override
      public void onSuccess(AccessToken token) {
        findById(token.getUserId(), this);
      }
  }

  // Usage:
  loginUser(email, password, new GetLoggedInUserCallback() {
      @Override
      public void onSuccess(User user) {
        // etc.
      }
      // onError
  });
  ```