Refactor TestRunner import to `get_runner` helper

This helper is available since Django 1.1.
Eliminate the need for a test_settings.py file.

Currently the code rely on `DJANGO_SETTINGS_MODULE` to setup Django settings to run the tests.
Implementing the function `configure_settings` we make `runtests.py` self contained.
However, it still is possible to override the programmed settings:

```bash
DJANGO_SETTINGS_MODULE='path.to.some_settings' ./runtests.py
```
Prepare to add more complex command arguments.
Add --use-tz flag to test model_mommy with timezone aware datetime.
Add tox to the project
Configure tox environments

* django1.4 (*USE_TZ=False*)
* django1.4-timezone (*USE_TZ=True*)
Update README testing instructions
Merge branch 'tox'