Add backends for encrypted storage, scrypt
We need an encrypted storage format for private keys that is better than openssl's armor, once we start storing ed25519 private keys (optionally encrypted).
We should also use a better passphrase-based-key-derivation function than we have now. scrypt isn't my favorite, but until the PHC is done, it's probably a good choice.
Once those are in, we can use scrypt in place of our current openpgp RFC2440 password-to-key function.