Initial version of sample code for the ndb modeling article.

This is an attempt to replace db with ndb in the following article:
https://developers.google.com/appengine/articles/modeling

Note that the original article was written in 2008, which was way before we released High Replication Datastore. Thus we need to introduce a strong consistent query by having parent keys for some entities, as well as migrating the code sample to ndb.
Mary should have the same key, so this line can probably be deleted.