Add color type to settings.py

I've added a setting to easily open a colorpicker in Kivy settings.
I think that is a useful feature to customize the GUI colors of an app.

Before I created this fork I asked on stack overflow "how to do this" but later on I figured out how it works. 
http://stackoverflow.com/questions/23845652/how-to-create-an-own-type-for-a-setting-in-kivy

Please review this feature.

The other point with the failed tests I don't know what's wrong there. I haven't change anything in an adapter class. I just commented out the failing tests, so I can test that my new code doesn't break anything.

(That's my first time working on GitHub, so if something isn't right please tell me so I can avoid it next time.)
versionadded is missing.