crypto/tls: support AES-GCM.

AES-GCM is the only current TLS ciphersuite that doesn't have
cryptographic weaknesses (RC4), nor major construction issues (CBC mode
ciphers) and has some deployment (i.e. not-CCM).

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13249044