Jfurr

This PR addresses changes in the 0.5.4 branch broke proxy models to polymorphic sublcasses
Consider the below model

```class Catalog_Item(PolymorphicModel):```
```class Kit_Item(Catalog_Item):```

```class Stored_Catalog_Item(Catalog_Item):
        class Meta:
            proxy=True

# stored_item will be a Kit_Item which is NOT a child class of Stored_Catalog_Item
stored_item = Stored_Catalog_Item.objects.get(id=1)
stored_item.get_real_instance()  <= FAILS
```

this fails because of the call here:
Stored_Catalog_Item


with 0.5.3 everything works as expe