Merge Kvis-Dev's 3.8-compatible fork

Compatibility with GNOME Shell version 3.8 was requested before (see #19). With Kvis-Dev's fork there is  a working implementation. [Merging it to vibou/vibou.gTile was requested before.](https://github.com/vibou/vibou.gTile/issues/16#issuecomment-26403068)

@vibou Please consider and discuss the changes here.
@Kvis-Dev When you make further changes to your fork; they will automatically show up in this pull request.