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.
(Moving convo to latest diff)

I totally appreciate the effort to make it generic and re-usable but that can distract from the sample and I'd imagine a good chunk of people reading won't know what `self.__class__` is.

Also, you said `self.parent_model_class` works but still have `self.__class__.parent_model_class` here and other uses of  `__class__` elsewhere.

RE: Slapping logic onto `MyModel`, I think the data / behavior separation is still quite nice, so you can leave it as-is.