Py3, django 1.7 for example app, wheel support, tox

Running the test is as simple as:
```bash
tox
```

If you want to run them manually:
```bash
cd examples/protected_downloads
python manage.py test sendfile
```

Tests work for django 1.4 -> 1.7, and python 2.6, 2.7, 3.2, 3.3, 3.4.
Dropped support for django 1.3, minimal django is now 1.4.2, because it is the minimal version that is effectively compatible with python 3.

Let me know what you think of this !