closes #103 add support for Django 1.7

Hi

Thanks for your great work on django-newsletter. I noticed this ticket has been open for a while and our application that uses newletter needs to be updated to Django 1.7, so I gave it a shot.

No more errors or deprecation warnings when running on Django 1.7, I also ran `python setup.py test` locally without errors.

Let me know if you see any issues and I'll be happy to revise. Thanks!