Add transports class variable to gunicorn workers to allow setting transports

Nginx is often put in front of other webservers, but does not support web-sockets.  This is generally fine, however if using the gunicorn worker, you cannot unset the websocket transport.  This causes a traceback about a missing wsgi.web_socket key.  There is already a closed issue about this.

Additionally, I added an Nginx specific worker class which sets the transport to xhr-polling.  This appears to be the only transport which is currently working.  It appears that there are several broken transports.