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 method fails with
```
  
{
 "error": {
  "code": 400,
  "errors": [
   {
    "domain": "global",
    "message": "Error parsing ProtoRPC request (Unable to parse request content: Message CombinedContainer is missing required field id)",
    "reason": "badRequest"
   }
  ],
  "message": "Error parsing ProtoRPC request (Unable to parse request content: Message CombinedContainer is missing required field id)"
 }
}
```
since you specify
```python
  @EndpointsAliasProperty(setter=IdSet, required=True, property_type=messages.IntegerField)
  def id(self):
```
in `ChildEndpointsModel`.