Reminds user to subscribe to face_detection node in order for the node to publish.

ROS users typically expect that a node of the subscribe/publish type (i.e. not an action server) will begin publishing once all its subscribed-to topics are available. Since this is not the case for face_detector, as in fact it waits to be subscribed to before publishing anything, we ought to add these reminder messages to notify the user of this irregularity.

Reminder message is sent in three different cases:
1. Upon initialization
2. If user disconnects from one of FaceDetector's outbound topics (RGBD camera)
3. "     "  (stereo camera)