Opt-in on message compression

Compression extensions are standardized by now.
For local websockets, the current behaviour of not compressing messages is optimal.
For use over the internet, compression is a necessary feature for e.g. interactive pixel graphics.

On the client side, the negotiated message compression is available in javascript using
websocket.extensions. A similar interface ought to be available on the server side.

The http request for a websocket contains the possibilities, e.g. from Chrome:
Sec-WebSocket-Extensions => permessage-deflate; client_max_window_bits

Ref. issue #76 - compression would currently be very effective even on local connections.