code: public final RequestToken revoke(Grant grant, String username, BaasHandler<Void> handler) {
          return grant(grant, username, RequestOptions.DEFAULT, handler);
      }
docs: Asynchronously revoke the access <code>grant</code> to this object from <code>username</code>.
  The outcome of the request is handed to the provided <code>handler</code>.
  
  @param grant    a non null {@link com.baasbox.android.Grant}
  @param username a non null username
  @param handler  an handler that will receive the outcome of the request.
  @return a {@link com.baasbox.android.RequestToken} to manage the asynchronous request