Add proxy_header capability to websocket protocol

[luxaritas]

So, after looking through this for a good few hours, it looks like the original implementation was fundamentally flawed, and unable to be addressed by #19 - in short, the self.request variable never got set, and the only place I could see to set it would have been in WebSocketsResource.render/_WebSocketsFactory.buildProtocol (as _WebSocketsFactory.buildProtocol would have to accept the request argument even if it were to be passed on to PeerOverrideFactory.buildProtocol), which didn't seem smart.

I'd definitely like to be double checked on my changes here, and if there is a way to make the original implementation work, I'd be down for that (would it make sense to subclass _WebSocketsFactory to do this or something?)