Fix test behaviour on Django < 1.6 and add Django 1.6 testing

* Django 1.6 is pretty widely used now, so it's worth testing on that version as well as 1.4 and 1.5.
* Most of the tests that are being run by Travis or `python manage.py test` belong to Django or OpenID, with only six tests actually built into `django-admin-sso`.
* DiscoverRunner can fix this for us, but it's not built into Django < 1.6, so we need to manually import it.  The import hacking has been isolated in a test runner file (which also makes it easier to expand the runner later).
* Move tests.py into the newly created test folder alongside the test runner and rename it to something more descriptive.
* Add `coverage` and make Travis report it.