Add support for Django 1.6

Three changes required:

 - BooleanField no longer defaults to False
 - filter(date__month=..., date__day=...) is now timezone aware, so
   a timezone needs to be specified in the settings
 - it now seems to require pytz, probably because of timezone
   awareness
Merge pull request #109 from bfirsh/add-support-for-django-1.6

Add support for Django 1.6