Stop recommending Tornado as a WSGI server

Tornado's WSGI server is less scalable than the alternatives and only really makes sense when you have a specific need to run both WSGI apps and Tornado-native apps in the same process. This recommendation has been an ongoing source of confusion, e.g. http://stackoverflow.com/questions/26015116/making-tornado-to-serve-a-request-on-a-separate-thread.

Fixes #986.