Add stopping capability

Currently, there's no way to stop listening. AWS.Request exposes an abort() that can be used to stop the queue. I'd make a pull request, but I don't drink Coffee.

Maybe store active requests returned from self.client.receiveMessage, and expose a disconnect() method on the queue?

Btw, love your use of the async module. i don't leave home without it.

:+1: to stopping capability.

Love to have the ability to pause() or stop()