Document gen_test change.
Point all links to the 'demos' directory to the stable branch instead of master.
Fix Tornado on Google App Engine.

Imports of tornado.ioloop have crept into more places, so the old
app engine policy of never importing these modules doesn't work.
Instead, we add guards around imports of unavailable modules
(fcntl, ssl, multiprocessing) so that everything is at least importable.

Closes #1059.