Enabled timezone support in Django.

This commit enables USE_TZ in settings.py. Additionally, the pytz
library has been added to requirements.txt.
Epoch fields now return timezone-aware datetimes.

This is done by explicitly marking datetimes returned from an
EpochField as UTC dates. In the templates, Django automatically
converts aware dates to localtime.
Fixed tests by always using aware datetimes
Fixed old usages of native datetimes