Separate OpenPGP model API from struct storage

[Casey Marshall]

The structures in pgp_types.go should be abstracted into interfaces. Basically, replace the names PubKey, UserId, UserAttribute, Signature, etc with interfaces and getter methods.

Underlying struct implementations can then adapt more closely with the backend model representation -- Mongo backend model can differ from PostgreSQL, go.crypto/openpgp structures can be used directly as a backend store instead of duplicating the data.

[Casey Marshall (cmars)]

This went down a little differently than I'd planned, but the OpenPGP models have definitely been refactored, and there's now a clear distinction and relationship between RFC4880 and the database representation.