Ignore heartbeat messages

lib/server.js

@@ -101,6 +101,8 @@ app._configureWS = function(socket, key, id, token) {
           dst: message.dst,
           payload: message.payload
         });
+      } else if (message.type === 'HEARTBEAT') {
+        // Ignore - nothing needs doing here.
       } else {
         util.prettyError("Message unrecognized");
       }