Django 1.4 timezone support with aware datetime with naive datetime

[euricojardim]

Bugfix with Django 1.4 while comparing aware datetime with naive datetime while executing send_newsletter command.

For more information see https://docs.djangoproject.com/en/dev/topics/i18n/timezones/

[yakky]

To support non-tz aware django 1.4 setup, there should be a check of settings.USE_TZ. Please check above commit