Check number of subscribers before publishing marker

This checks whether a node has subscribed to the marker topic.
The marker is not published until someone subscribes.

Related [answers.ros.org](http://answers.ros.org/question/199299/why-do-i-need-to-enter-a-loop-to-display-a-marker/) topic.
It might also be good to do that check before the `ROS_WARN_ONCE` call. So that if you `ctrl-c` quickly it doesn't print the message before exiting.