appengine example now uses new database library ndb instead of db

It's unclear whether ndb Entity id correspond to the db Entity key name or not, docs are not clear about it (https://developers.google.com/appengine/docs/python/ndb/modelclass#Constructor). I've tried different solutions but 

- define id property and query with id as argument works
- NOT define id property and query with id as argument NOT works
- define id property and use get_by_id for retriving entity NOT works

Anyhow for changing least as possible I retained the id name and adopted the first from previous solutions.