Update WSGI functions to take necessary arguments

Update WSGI functions to take necessary arguments

* gunicorn.py needs to initialize mailman once, not per request.
* make_application takes environment and response arguments, which it passes to
  the callable WSGI application (in this case, the instance of the RootedAPI
  class).  (wsgiref does something similar, but wsgi_server only needs a copy
  of the application, and it passes the same two arguments to that object
  internally.

Note, in my opinion wsgiapp.py's make_application has a misleading name, since
it doesn't behave like a normal make_application (it's not invoked by a WSGI
server). I've left it as-is, though, since it is public.