Add the 'mod_stun_disco' module, which allows XMPP clients to discover
STUN/TURN services and to obtain temporary credentials for using them as
per XEP-0215: External Service Discovery. The temporary credentials
handed out to clients have the format described in:
https://tools.ietf.org/html/draft-uberti-behave-turn-rest-00
Also add the new module to the example configuration file.
Closes#2947.
