- Introduce exceptions to be raised upon transient and permanent delivery
failures, as specified by SMTP RFC. Depending on type of failure, return
either 451 or 554 reply code.
- When serialising a message, treat ValueError as a serialisation issue (and
try again to deliver in cleartext).
Introduce new Python modules:
- lacre.notify -- to send notifications from the cron script;
- lacre.dbschema -- to keep database schema definition as code (SQLAlchemy);
- lacre.repositories -- to define key and identity repositories with high
level APIs that we can then use elsewhere.
Also:
- rework GnuPG.add_key to return fingerprint so we can use it in the cron
script;
- rename cron-job's logger name, replacing dash with an underscore as logging
module doesn't like dashes.
