Switch easily between light/dark color settings

Use a global variable config.env to define user preferences (light/dark).

Change :
- the color settings (Xresources file)
- the background image (also used as the screensaver)
Appearance module.

Deal with appearance as a loaded rc module.

Change some settings to adapt the systray (taken from V. Bernat).
Add a cmus widget and controller

The widget display the state of cmus and several infos.
The controller allows to play/pause/go next/go previous.

Stolen from Robin Hahling
http://blog.rolinh.ch/linux/un-widget-controleur-pour-le-lecteur-audio-cmus-pour-awesome-wm/
Slight change in the xterm color settings