message: check for existence of callback before calling it
diff: @@ -240,9 +240,9 @@ function Sparkbot(configuration) {
  delete ciscospark_message.channel;
  controller.api.messages.create(ciscospark_message).then(function(message) {
  - cb(null, message);
  + if (cb) cb(null, message);
  }).catch(function(err) {
  - cb(err);
  + if (cb) cb(err);
  });
  };
  