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