code: public RequestToken login(String regitrationId, int flags, BaasHandler<BaasUser> handler) {
          BaasBox box = BaasBox.getDefault();
          if (password == null) throw new IllegalStateException("password cannot be null");
          NetworkTask<BaasUser> task = new LoginRequest(box, this, regitrationId, flags, handler);
          return box.submitAsync(task);
      }
docs: Asynchronously logins the user with password and registrationId obtained through gcm.
  The handler will be invoked upon completion of the request.
  The request is executed at the gien priority.
  
  @param regitrationId the registrationId
  @param flags {@link RequestOptions}
  @param handler       an handler to be invoked when the request completes
  @return a {@link com.baasbox.android.RequestToken} to handle the async request