#5 Allow for hot-plugging PA-1 device

[David Costanzo]

If you start gnopa and the device is not plugged in, it
prints a warning to stderr. As far as I can tell, the
only way to recover from this is to plug the device in
and restart gnopa.

As a feature request, I would like to be able to plug
and unplug my PA-1 device while gnopa is running.
Ideally, gnopa would provide some visual feedback when
the device is plugged in or unplugged (either a popup
dialog box, or a status icon).