This introduces new APIs.
- DialConfig can open client connection using Config, so user can specify protocol version, tls.Config if necessary.

- Message can be used to send/receive text/binary data in a frame.
- JSON can be used to send/receive JSON data in a frame.

R=golang-dev, adg, rsc, m, tarmigan, raul.san, yohcop
CC=golang-dev
https://golang.org/cl/4635084