Added GTalk XMPP support

If the user specifies an additional xmpp_notify_domain, then this
overrides the xmpp_notify_server setting for authentication.
This is needed to connect to GTalk (now Hangouts) by XMPP for user
john.doe@examplegoogleappsdomain.com :

xmpp_notify_user=john.doe
xmpp_notify_domain=yourgoogleappsdomain.com
xmpp_notify_server=talk.google.com
xmpp_notify_pass=securepassword