Gnome Shell 3.8 port

[genosse-einhorn]

please note that this also removes compatibility with 3.4 and 3.6, you might want to reimplement my changes if you want to keep it.

[axeldavid]

Since @rgcjonas changed the version number in metadata.json (replaced "3.4", "3.6" with "3.8") these changes won't affect users with older versions of Gnome on extensions.gnome.org. They will simply be provided with the latest version of this extension that is compatible with their Gnome version. The lack of compatibility with Gnome 3.4 and 3.6 will therefore not be a problem.

I've tested these changes on my machine (Fedora 19 with Gnome 3.8) and it works like a charm without any errors or warnings to the log.

@war1025 I find your gnome extension very useful and this update would be much appreciated :)