Fixed bug 1120: make the GNOME shell extension compatible with GNOME 3.8

The D-Bus Glib bindings, which were deprecated since GNOME 3.4, have been
removed in GNOME 3.8. This commit replaces them by the new Gio bindings.
Merge pull request #33 from sebastien-villemot/master

Fixed bug 1120: make the GNOME shell extension compatible with GNOME 3.8