* Update to color scheme building: each scheme only contains the variables, Gruntfile handles building both customizer and admin
stylesheets based on _admin.scss and _customizer.scss
* Stylesheet names have changed; theme/admin-colors.css for admin colors, theme/customizer.css for customizer stylesheet.
* Update customizer stylesheet to use color schemes's customizer stylesheet, if defined & exists (fallback to MP6 if not)
* Add a function `mp6_add_admin_colors` to help add schemes, which sets up the admin & customizer stylesheet paths based
on the theme slug (and therefore is internal-use only currently) 




git-svn-id: https://plugins.svn.wordpress.org/mp6/trunk@770416 b8457f37-d9ea-0310-8a92-e5e31aec5664