crypto/tls: implement tls-unique channel binding (RFC 5929 section 3).
Tested against GnuTLS and Python.

LGTM=agl
R=golang-codereviews, agl, ashankar
CC=agl, golang-codereviews
https://codereview.appspot.com/117100043

Committer: Adam Langley <agl@golang.org>