Fix proposal for get_real_instance_class() for proxy models

This PR addresses changes in the 0.5.4 which branch broke ```get_real_instance_class()``` for proxy models of polymorphic base classes

Consider the below model

```
class Catalog_Item(PolymorphicModel):
class Kit_Item(Catalog_Item):

class Stored_Catalog_Item(Catalog_Item):
    class Meta:
        proxy=True


# grab a Stored_Catalog_Item object that refers to a Kit_Item
stored_item = Stored_Catalog_Item.objects.get(id=1)
stored_item.get_real_instance() #  <= FAILS

# This works
stored_item = Catalog_Item.objects.get(id=1)
stored_item.get_real_instance()  # <=  FINE
```

this fails because of the call here 
https://github.com/chrisglass/django_polymorphic/blob/master/polymorphic/polymorphic_model.py#L113: ```issubclass(model, self.__class__)```  where ```model=Kit_Item``` and ```self.__class__=Stored_Catalog_Item``` because ```Kit_Item``` isn't a child class of ```Stored_Catalog_Item```, but rather``` Catalog_Item```.

The fix I propose is actually pretty trivial in that it simply checks to see if ```self.__class__``` is a proxy model for a django polymorphic base class.:
```
if model is not None  \
and not issubclass(model, self.__class__) \
and not issubclass(model, self.__class__._meta.proxy_for_model):
```