title: Subscribe to a simple ros node in rqt
body: <p>I'm trying to subscribe to a very simple ros topic from within an rqt plugin. It seems like the subscription works, but my Callback function does not get called. </p>
  
  <pre><code>Subscription::Subscription(QString topic)
  { 
     ROS_INFO("Subscribing to topic: [%s]", topic.toLocal8Bit().constData() );
  
     sub = n.subscribe(topic.toLocal8Bit().constData(), 1000, &amp;Subscription::chatterCallback, this);
  }
  
  
  void Subscription::chatterCallback(const std_msgs::String::ConstPtr&amp; msg)
  {
     ROS_INFO("I heard: [%s]", msg-&gt;data.c_str() );
  
     emit messageReceived(msg-&gt;data.c_str() );
  }
  </code></pre>
  
  <p>This should actually be very simple. What am I missing?</p>