REST runner WSGI server support
The REST runner should be able to be run from a production WSGI application server, instead of the reference implementation.
From a mailman-developers thread I started, https://mail.python.org/pipermail/mailman-developers/2015-July/025082.html Barry created an experimental branch for running WSGI under gunicorn. I made changes necessary for it to run, and I think it should be merged in so that others can use it or file bugs on it.
It's supplemental to the existing code. There are no tests for it yet.
(Barry, I've just now rebased both commits to origin/master, including yours.)