Add websocket.Server to configure WebSocket server handler.

- Config.Header is additional headers to send, so you can use it
  to send cookies or so.
  To read cookies, you can use Conn.Request().Header.
- factor out Handshake.
  You can set func to check origin, subprotocol etc.
  Handler checks origin by default.

Fixes golang/go#4198.
Fixes golang/go#5178.

R=golang-dev, mikioh.mikioh, crobin
CC=golang-dev
https://golang.org/cl/8731044