Changed methods used for traversing subclasses and superclasses of a given model.

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