Django 1.7 support

* ./runtests.py will now run when Django 1.7 is installed. (I tested back to Django 1.4)
* Cleared up the Django 1.8 warnings.
* Tests will run in Django 1.7. Classes that threw known Django issues, now pass, and print a warning.


This exception is too broad here.  That could anything, a `Model.DoesNotExist` for example.  You should write something like:
```python
def get_queryset(self, request):
    parent_self = super(PolymorphicParentModelAdmin, self)
    if hasattr(parent_self, 'get_queryset'):
        return parent_self.get_queryset(request)
    return parent_self.queryset(request)
```