crypto/x509: allow ECDSA public keys to be marshaled.

The public key serialization from CreateCertificate is factored out to be
used in MarshalPKIXPublicKey.
Testcode with one P224 ECDSA keypair has been added.

R=golang-dev, agl
CC=agl, golang-dev
https://codereview.appspot.com/13427044

Committer: Adam Langley <agl@golang.org>