Settings GUI now contains a checkbox for UPnP and auto starts/stops the
thread when changed.
Default UPnP socket timeout decreased for faster thread
starting/stopping
Fixes Bitmessage#373
Fixes Bitmessage#79
Fixes#4
Needs to be turned on in the config file (keys.dat) by adding "upnp =
true" into the main section
It needs some cleanup (e.g. if you have multiple routers, it will not
correctly handle failure of one of them)