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.
Since this is identical to the app config for `basic`, can you make this a symlink as in

https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/master/examples/basic_with_auth/app.yaml