crypto/x509: unwrap Subject Key Identifier
crypto/x509: unwrap Subject Key Identifier

RFC 5280, 4.2.1.2 says:
  SubjectKeyIdentifier ::= KeyIdentifier
  KeyIdentifier ::= OCTET STRING

Previously, we were failing to unwrap the second level of OCTET STRING
encoding.

Fixes issue 993.

R=rsc
CC=golang-dev
http://codereview.appspot.com/1917044

wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old