UrfDevice: Properly release resources
UrfDeviceOfono: Properly release resources
UrfOfonoManager: Fix ofono re-start case

Urfkill was crashing when ofono re-started because resources where not
properly released when a ModemRemoved signal was received. Also, the
id counter was not being reset and urf_device_ofono_get_modem_path was
being called with a wrong argument.
Merge pull request #16 from alfonsosanchezbeato/handle-ofono-restart

Better handle ofono restarts by properly cleaning up resources.