Fix NewsletterSubscriber date_subscribed

This fixes issue #43:

The `NewsletterSubscriber.date_subscribed` default is `datetime.datetime.now()`, which has two problems:

1. The time is fixed when the module is loaded (it's the same for every added subscriber during one run of a script, or until the webserver is reloaded)
2. The date/time is not timezone-aware, causing warnings when timezones are enabled in Django 1.4 and newer
Update timezone fix to work with Django 1.3

The fix for issue #43 in pull request #44 only works in Django 1.4 and earlier. Pennyblack needs to work with Django 1.3, so try to import `django.utils.timezone.now`, but if that fails then fall back to datetime.now
Merge pull request #44 from nickburlett/official-develop

Fix NewsletterSubscriber date_subscribed