Added child_enpoints_model example.

As per https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/issues/110#issuecomment-60832527 I have added the example for simple parent child relationship.
This is overly generic for a sample, we should just use `MyParent` here. I don't expect many will know what `__class__` is. Also note that `self.parent_model_class` will return a class variable.