Support for WebSockets in Server interface

[wicknicks]

net/http added support for websockets in v1.12 (discussion). The change however requires applications to use an additional interface (Hijacker) to 'hijack' the connection and access the I/O channels. When applications use go-kit, the http.ResponseWriter is replaced by an interceptingWriter here: https://github.com/go-kit/kit/blob/v0.10.0/transport/http/server.go#L99. This only embeds the original http.ResponseWriter. To open up support for websockets, we needs to embed the Hijacker interface as well.