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.
This should probably also be conditional on `ros::ok()` or better yet, in the while loop just check and exit if it is not, something like this:

```c++
ROS_WARN_ONCE(...);
if (!ros::ok()) {
  return 0;
}
sleep(1);
```

The reason for this is so that when it is waiting for you to subscribe and you ctrl-c the program it will actually exit without you have to escalate to ctrl-\.