Support Django 1.7rc1

Fix codes to support Django 1.7rc1 and update tox to test followings:

- Django 1.3.7 with Python 2.6, 2.7
- Django 1.4.13 with Python 2.6, 2.7
- Django 1.5.8 with Python 2.6, 2.7, 3.2, 3.3, 3.4
- Django 1.6.5 with Python 2.6, 2.7, 3.2, 3.3, 3.4
- Django 1.7rc1 with Python 2.7, 3.2, 3.3, 3.4

This commit close #26
Bump version