Change background and themes links to point to mate-desktop.org. 

Hi,

As discussed in IRC I've created some landing pages on mate-desktop.org for Backgrounds and Themes.

 * [Backgrounds](http://mate-desktop.org/backgrounds)
 * [Themes](http://mate-desktop.org/themes)

This pull request simply changes the broken links pointing to art.gnome.org with links to the above landing pages. Fixes #85 and #87.