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 worthy.
Agreed (damn you PEM serialization encryption). I'm reluctant to add different serializers between public and private; what do people think of just raising an exception if you use anything but `NoEncryption` when calling `dump_pem` on a public key? Not the most elegant, but less objectionable than `PublicPKCS8` and `PrivatePKCS8` classes...