Add zzz classes and light version

This pull request:

- add to img/sprites/shop_sprites.png a light version of the 'zzz' image to show when user is resting. Needed to show on the mobile app menu since it has a dark background. I'm not very good at graphic so if there's somethign that needs to be changed (like the color), let me know.

- add the classes 'zzz' and 'zzz_light' to css/shop_sprites.css since the former was missing and the latter is now necessary