added reply functionality

Hi dude,

I was using your module at work, but we needed reply functionality so I extended your code. Now both server and client can choose to set set a reply function and it will be executed accordingly.

Basically this adds ack-handling and callbacks.

You need to adapt the code if you refuse to update vertx: JsonObject.containsField() is not present in older versions of vertx, but a try-catch-block would probably do the trick, too. (DefaultSocketIOSocket.java, line 391)

Best regards,
Robert