Custom test runner support

compat.py implementation changed to use `django.test.utils.get_runner`. This allows users to override the test runner using the `settings.TEST_RUNNER` configuration. Wrote a test to verify that it works. Also, none of the other tests were broken because of this change. (There are some issues in Travis with PyPy builds and also with builds which include Django-master but they existed before this PR as well.)

pelme/pytest_django#121