queue incoming messages until session is setup

with this patch the test @silverbucket wrote for #90 passes. Also I can't reproduce the bug with the browser anymore either.

As far as I understand, this wouldn't solve the [related issue](https://github.com/Worlize/WebSocket-Node/issues/91) in WebSocket-Node, but it seems to me that this was never the problem here.
The "message" handler for the connection was only installed by the Dispatcher after Dispatcher.session.get()'s promise was fulfilled, giving the connection time to receive data in between that was then never received by sockethub.