code: public RequestToken login(BaasHandler<BaasUser> handler) {
          return login(null, RequestOptions.DEFAULT, handler);
      }
docs: Asynchronously logins this user.
  The handler will be invoked upon completion of the request.
  
  @param handler an handler to be invoked when the request completes
  @return a {@link com.baasbox.android.RequestToken} to handle the async request