Publishing multiple face bounding boxes to ROS on '/face/<id>' topics

Published topics are named **faces/\<id>**, where **\<id>** is an integer corresponding to a detected face like in the flowgraph document. Although I'd recommend publishing all of the face bounding boxes as a list on a single topic, since when old faces disappear and new ones are detected, the topic list is quickly cluttered with once-published but never again used topics. We could also avoid opening another communication channel for **new face** events this way.

Also, colon (":") character as specified in the flowgraph doc is not allowed in a topic name.