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.
Prefer some information about expected entity keys in the docstring.