Should we prefer celery tasks rather that threads ?

I'm referring to the [`EmailThread`](https://github.com/dominno/django-moderation/blob/b357fec29011c7c973a9f75722caef219e5d817f/moderation/moderator.py#L15) class, we probably should change that for django celery tasks if that's available, otherwise use the main thread to perform it, don't you think ?

Moreover, it broke the test suite sometimes for strange reasons...

I'm willing to make to changes in PR if you agree or complete this thought :)