Retrain autotaggers periodically (#536)

Try to fix #536. This should implement retraining autotaggers periodically. I'm open to feedback, especially on the `@classmethod`. Making an instance seems a bit ugly, however it seems that running a `Command` also does that. Maybe `get_keywords` with it's `rcache` could be integrated into `_retrain()`, then it doesn't depend on `self` anymore. I'm however not quite sure what's going on there.

(what about `autotagger_sb.py` btw?)