Use Tox for multiple environment test

With this patch it's possible to test the project against Django 1.4 with and without timezone aware datetime.

What was done:

1. Refactor runtests.py not to depend on external settings file;
2. Refactor runtests.py to Django get_runner;
3. Add and configure Tox for Django 1.4
4. Add command line option to runtests.py to activate `USE_TZ`;