Some OCSP responses (notably COMODO's) don't use an intermediate certificate to sign OCSP responses so this change allowed that. Additionally, a CreateRequest function is added for creating OCSP requests. This change makes one API change: SerialNumber becomes a *big.Int to match crypto/x509. The original code dates to before encoding/asn1 could cope with big.Ints and using a []byte was a workaround for that. R=golang-dev CC=golang-dev https://golang.org/cl/10402043