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.
You can specify different `request_fields` depending on the request. It's not something globally set.

The question is if you want users to be able to insert their own chosen IDs (which here would overwrite existing).