code: public RequestToken logout(BaasHandler<Void> handler) {
          return logout(null, RequestOptions.DEFAULT, handler);
      }
docs: Logouts the user from the server. After this call completes no current user
  is available. {@link BaasUser#current()} will return <code>null</code>.
  
  @param handler an handler to be invoked upon completion of the request
  @return a {@link com.baasbox.android.RequestToken} to handle the async request