Enforce creation of revisions

[matthijskooijman]

It would probably be good to enforce that a revision is created for every change to the database (e.g. to prevent forgetting create_revision somewhere).

The best way for this is probably a pre-save hook that tests whether a revision is active. Might be nice if django-reversion would offer this builtin, but that might need some more convincing, see: etianen/django-reversion#829