_getVolume: change the regexp to make it work when the volume level is 100%
_setVolume: remove the space between the volume value and '%', otherwise
            amixer set interprets the value as a hardware value instead of
            a percentage
Make the extension work with gnome-shell 3.10, PanelMenu.SystemStatusButton
 is gone
destroy: move this.parent() to the end of the destroy function, otherwise
         disabling the extension throws a window manager warning about gsignal
Use symbolic icons
Modify metadata.json