Add ping-pong (heartbeat) to detect if 1 side of a peer connection closes

Add ping-pong (heartbeat) to detect if 1 side of a peer connection closes.  This fixes issue https://github.com/uProxy/uProxy/issues/151 (so that if a client closes, the server's UI can remove the X is proxying through you text).  This is also necessary to have in place for fixing issue https://github.com/uProxy/uProxy/issues/178 (that will require some new UI)

Tested with manually and with "grunt test"