Added cursors for rudimentary pagination

These may be elucidating:
https://developers.google.com/appengine/docs/python/datastore/queries#Retrieving_Results
https://developers.google.com/appengine/docs/python/datastore/queryclass#Query_cursor
http://p.ota.to/blog/2013/04/pagination-with-cursors-in-the-app-engine-datastore/