PyPI Distribution Sub-Package Fix

This resolves #4 by including the `tests` and `extra` packages with the PyPI distribution.

There were two ways of fixing this:

1. Add `xmlrunner.extra` and `xmlrunner.tests` to the `packages` list in `setup.py`, and
2. Add import statements to appropriate `__init__.py` files leaving breadcrumbs for `distutils` to follow and implicitly include the sub-packages in the distribution installation.

I included both of these fixes, the first because "explicit is better than implicit" and the second to make it easier to import these sub-packages.

I also took the opportunity to make `setup.py` compatible with PEP8 (just removing the spaces between the keyword arguments in the `setup()` function) and to make `djangotestrunner` a fully usable out-of-the-box module. Previously, it seemed to have been an example script, even including the line

    os.environ['DJANGO_SETTINGS_MODULE'] = 'mysite.settings'

which was too generic to be applied to an actual Django project environment. The module can now be used as mentioned in the README (I think).

Let me know if anything is not to your liking.