Compatibility with Gnome Shell 3.8 and 3.8.1

Inspired in
https://github.com/eonpatapon/gnome-shell-extension-calc

Should keep working with Gnome Shell 3.6 (not tested)
Added repository URL to extension metadata.

The URL should now (hopefully) show on the Gnome Shell extensions catalogue as
the "Extension Homepage".
Merge pull request #4 from vklj/master

Upgrade to 3.8