Test that proxy models derived from PolyMorphicBase classes
can call get_real_instance() and get_real_instance_class()

Includes:
def test_proxy_get_real_instance_class(self):
Merge branch 'jf/polymorphic' of https://github.com/gnulnx/django_polymorphic into gnulnx-jf/polymorphic
Improved description of test_proxy_get_real_instance_class unit test

Camp life rule: leave things better then you found them.