Start lamson mail logger with supervisord (fixes #233)

This uses a custom method of starting lamson in logging mode, which does
not daemonize lamson [0].

Mailserver logs end up in `parts/mailserver/logs/logger.log` instead of
`var/log/lamson.log`. This could be changed by calling the stuff from
`lamson.utils.make_settings` manually, but I think it's not worth it.

[0] https://gist.github.com/nidico/5076333
Properly install and use lamson locally
Merge pull request #256 from nidico/feature-233-start-lamson-with-supervisor

Start lamson mail logger with supervisord (fixes #233)