Change wallpaper extension

[Javier Goday <jgoday gmail com>]

Hi, i just made a simple extension to set the current background from
the overview mode.

For now, it has two different image providers:

1) Local images ( search images under local directories, by default
/usr/share/backgrounds and ~/Pictures)

2) Deviantart wallpapers  ( using http://backend.deviantart.com syndication )

It's my first try, so i would like to here comments and advices.

Sources: https://github.com/jgoday/gnome-shell-extension-wallpapers

Thanks.