Patch for problem with USE_TZ = False

This patch fixes https://github.com/celery/django-celery/pull/190 and https://github.com/celery/django-celery/pull/197.

I added a class decorator @fix_timestamp for models in djcelery. I think this is not a optimal approach to fix current issues in MySQL, but looks better than https://github.com/celery/django-celery/pull/197. :)