Implement Hijacker() in web.ResponseWriter for websockets

[Phea Duch]

Unfortunately you can't use Gocraft's Handler for websockets as it does not implement the required Hijacker interface. Passing web.ResponseWriter will fail at this line:

https://github.com/gorilla/websocket/blob/master/server.go#L134

[Jonathan Novak]

@phea Send a PR if you'd like this support.

[Jonathan Novak]

@phea I implemented this.