Add AddSignerInfo

[hamano]

Hi,
I'm trying to sign with PKCS11 API and format to CMS.
The interface sd.AddSigner(cert, pkey, config) isn't allow to sign with hardware token.
I just add new interface AddSignerInfo(cert, signerInfo)
also I published SignerInfo struct and Cert2issuerAndSerial()

Usage:

toBeSigned, _ := pkcs7.NewSignedData(content)
attrs, hashed, _ := toBeSigned.HashAttributes(crypto.SHA1, pkcs7.SignerInfoConfig{})
sigunature := SignWithHardwareToken(crypto.SHA1, hashed)
signerInfo := pkcs7.SignerInfo{
    Version:                   1,
    AuthenticatedAttributes:   attrs,
    EncryptedDigest:           signature,
    ...
}
toBeSigned.AddSignerInfo(cert, signerInfo)

Thank you.

[jvehent]

Fwiw, I tried merging this patch on mozilla-services/pkcs7, but it conflicts with the rewrite from #14.

[hamano]

@jvehent mozilla-services/pkcs7 already support HSM, so no need to merge the patch.
I'm going to use mozilla-services/pkcs7 thanks!