Make compatible with Python 3

Dropped support for Python 2.6 and Django 1.3, if you require support
for these or lower you will need to use an older version of
django-mail-queue. We currently support the following:

 + Python 2.7 with Django >= 1.4
 + Python 3.3 with Django >= 1.5
Merge pull request #30 from Matt-Stevens/feature/python_3

Make compatible with Python 3