Introduce bouncy.

 o Refactor domain layout to use PublicKey and PublicKeyMeta instead of
   PGP* classes. PublicKeyMeta is what you get from search listings,
   containing bare minimum data for user search listings.

   PublicKey extends PublicKeyMeta, and represents a real downloaded
   public key. It's backed by PGPPublicKeyRing from bouncy.

 o Still need to verify downloaded keys to ensure they are who they
   claim to be.