Mysql db fixes

Celerycam was raising "ValueError: MySQL backend does not support timezone-aware datetimes." because the last_heartbeat field was TZ aware.

I've applied the same fix from  https://github.com/celery/django-celery/pull/153 to all of the other DateTimeFields in the celery models. This is almost certainly overkill, but it works.

I'm running:
* celery 3.0.11
* django-celery 3.0.11
* django 1.3