Allow selenose.cases to be imported with Django installed but no config

If you have Django installed, but don't have a Django settings module to import, importing `selenose.cases` raises `ImproperlyConfigured` when `django.test` is imported.

This change makes sure you have both Django installed *and* valid settings before trying to subclass `django.test.LiveServerTestCase`.

It may be that the problem is actually with Django, and `import django.test` should work without valid settings. What do you think?