Fixes South Datamigration incompatibility.

Basic idea:
Properly inheriting from DjangoTestSuiteRunner and calling parent methods.

Behaviour change:
Feature tests aren't sorted before default tests; if unittests fail,
integrationtests have higher chance of failing too and  --fail-fast should fail
as fast as possible.

Also:
 - removes/comments unused imports
 - removes superfluous call to unittest.TestCase.__init__
   DjangoBehaveTestCase is a django.test.LiveServerTestCase is a
   unittest.TestCase, so it should be called already if we call super().init.
Automated merge with git+ssh://github.com:rwillmer/django-behave.git