Add scope to retrieve decrypted values directly from DB (only for PostgresPgp encryptor at the moment)

Added `decrypted` scope that allow you to retrieve decrypted values from database in one query (in old way, when you had `n` encrypted attributes, you had `n+1` database queries when retrieving one record - one `SELECT` query for record and `n` queries for decrypting encrypted attributes). With `decrypted` scope you would fire only one query.

(extracted from #19)