CMS support

[groner]

Starting a list of requirements for supporting CMS.

• ASN.1
• Asymmetric Encryption/Decryption (enveloped-data)
• Symmetric Ciphers (enveloped-data)
• Asymmetric Signing/Verification (signed-data)
• Asymmetric key loading

This is far from complete. I'm avoiding X.509 in my current project, for now, but I guess we'll want that eventually.

[groner]

OpenSSL supports some content-types, but I think (need to check) they are mostly related to X.509 use cases. I think having a python library for composing other ASN.1 records would be useful.

[reaperhulk]

#936 added some of the things we'll need as well.

[reaperhulk]

We're cleaning up our backlog and we're going to prioritize PKCS7 and (eventually) SMIME over the wider CMS profile. Closing this for now.