code: public RequestToken login(String registrationId, BaasHandler<BaasUser> handler) {
          return login(registrationId, RequestOptions.DEFAULT, handler);
      }
docs: Asynchronously logins this user with password and registrationId obtained through
  gcm. The handler will be invoked upon completion of the request.
  
  @param registrationId a registration id for gcm
  @param handler        an handler to be invoked when the request completes
  @return a {@link com.baasbox.android.RequestToken} to handle the async request