Improve detection of disconnection

[Massimiliano Mirra]

It takes several minutes until a severed disconnection (by gateways, firewalls, and whatnot) is detected. Other clients seem to do this more reliably.

Find out techniques for testing connection/disconnection other than relying to the underlying OS and implement them. A possible one is periodically sending an iq ping (http://www.xmpp.org/extensions/xep-0199.html), starting/resetting a timer, and declaring connection dead if a reply isn't received within a certain time.