Feature decrypt mails

This is both a pull request and a rfc/heads up/hey I'm going somewhere with this, am I going in the right direction-thing.

I sat down and hacked OpenPGP parsing and decryption into alot and it turned out to be not as bad or hard as I imagined. With this tiny patchset I've been able to read an encrypted mail created by alot 0.3.4, haven't tested mails from any other MUAs yet. Comments?