Create App for Testing

Ass discussed in #8, tests will now use an app instead of mock.

- [x] Move tests into `test_tracking.py` and `test_redirects.py` and `test_integration.py`
- [x] Create `models.py` for testing
- [x] Create `urls.py` for testing
- [x] Add app to `INSTALLED_APPS`
- [x] Refactor `test_tracking.py`
- [x] Add `test_integration.py` to REALLY make sure it works