Added a test case for testing parent_link and related_name links to parent.
All tests pass should pass now (at least they do for me on django 1.5 and python 2.7)
Now uses model._meta.parents to determine superclasses. _meta.parents is a dict of superclass: field_to_superclass pairs.
By using the field name of field_to_superclass, we can work out the field to use the django_polymorphic accessor on, even if a user-specified OneToOneField to parent is is used (with parent_link=True) to get to the parent in the inheritance hierarachy.
For subclasses, the path to the subclasses can be manually specified by the user if they use a 'related_name' on the OneToOneField from the subclass to the superclass. I have changed the code to also support 'related_name' if present
hottwaj