Simplify keepalive resets

Websocket resources should have their keepalive timers reset whenever a
message comes in. This is a nicety that slightly reduces the amount of
traffic we send when actively messaging.

Previously this was handled by MessageReceiver, but it's a bit cleaner
to just have the WebsocketResource add an extra 'message' event handler.

// FREEBIE

libtextsecure/websocket-resources.js

@@ -134,8 +134,9 @@
                 path       : opts.keepalive.path,
                 disconnect : opts.keepalive.disconnect
             });
-            this.resetKeepAliveTimer = keepalive.reset.bind(keepalive);
-            socket.addEventListener('connect', this.resetKeepAliveTimer);
+            var resetKeepAliveTimer = keepalive.reset.bind(keepalive);
+            socket.addEventListener('connect', resetKeepAliveTimer);
+            socket.addEventListener('message', resetKeepAliveTimer);
             socket.addEventListener('close', keepalive.stop.bind(keepalive));
         }