Support for traditional OpenSSL and PKCS8 RSA private key serialization

This temporarily uses the backend to do it until the alternate approach is ready. DSA/ECDSA and public key support will follow if this is deemed 
This doesn't actually have it's own encryption, it's PEM encryption there, and you can do PEM encryption with PKCS8 keys as well. I think we should pull the encryption out of here or something?