sync part2-usingusers branch with documentation https://developers.google.com/appengine/docs/python…
sync part2-usingusers branch with documentation https://developers.google.com/appengine/docs/python…

Reviewed in https://codereview.appspot.com/9834045/

Romzz/appengine-guestbook-python,artificechicago/camp2016,t9nf/appengine-guestbook-python,GoogleCloudPlatform/appengine-guestbook-python,bkeyvani/appengine-guestbook-python,jerarl/gcp-appengine-python,Romzz/appengine-guestbook-python,JonGal/appengine-ml-demo,thejeshgn/appengine-guestbook-python,wcyuan/weather-mailer,jeffgodwyll/appengine-flask-guestbook,ashwini0529/appengine-guestbook-python,bkeyvani/appengine-guestbook-python,adjohnson916/appengine-guestbook-python,JackConnor/portfolio,dmolson/appengine-guestbook-python,t9nf/appengine-guestbook-python,ashwini0529/appengine-guestbook-python,JackConnor/portfolio,GoogleCloudPlatform/appengine-guestbook-python,artificechicago/camp2016,wcyuan/weather-mailer,jerarl/gcp-appengine-python,dmolson/appengine-guestbook-python,thejeshgn/appengine-guestbook-python,adjohnson916/appengine-guestbook-python,jeffgodwyll/appengine-flask-guestbook,JonGal/appengine-ml-demo