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 
So only support encryption on PKCS8 for the moment and add a PEMEncryption() later when we switch to the alternate approach? I believe OpenSSL can only do PEM encryption on these traditional keys.