Naive datetime warnings

Running with Django 1.4 and USE_TZ=True, I get warnings like:

    local/lib/python2.7/site-packages/django/db/models/fields/__init__.py:827:
    RuntimeWarning: DateTimeField received a naive datetime (2013-03-20 15:02:45.949419)
    while time zone support is active.

Let's review rapidsms datetime usage with https://docs.djangoproject.com/en/1.4/topics/i18n/timezones/#time-zones-migration-guide in mind and see if we can make this work as expected.