Make testing easier

The `setup.py` changes allow this to work:

    python setup.py test

That's how I usually run tests since I don't always remember the various test module naming conventions that are used.

The tox file allows this to work for running tests with multiple versions of Django/Python:

    pip install tox
    tox

The gitignore ignores file clutter created by using `python setup.py test` (with missing dependencies), `tox`, and `coverage` (for testing code coverage).