Add support for Ring websocket API (WIP)

[weavejester]

This PR implements the Ring WebSocket API for this adapter. This is not intended to be merged yet, as the Ring WebSocket API is not yet stable, however I wanted to get some early feedback before the next version of Ring is released.

This PR will eventually close #29.

[yogthos]

Overall looks good, and pretty straight forward I think. Only thing I'd add is that it might be nice to add a helper like the following that would just let the users pass a map of functions as the public API:

(defn ws-listener [{:keys [on-open on-message on-pong on-close]}]
  {:ring.websocket/listener
   (reify wsp/Listener
     (on-open [_ sock] (on-open sock))
     (on-message [_ sock mesg] (on-message sock mesg))
     (on-pong [_ sock mesg] (on-pong sock mesg))
     (on-close [_ sock code reason] (on-close sock code reason)))})

[weavejester]

That's a good idea, and I've already implemented it 😉 . The ring.websocket namespace extends the Listener protocol to cover a map of functions.

[yogthos]

awesome, otherwise just ping me whenever new Ring version is out with ws support and we can push this change out along with it then. :)