Feature/lessvars

Pull request for editable LESS variables. This is actually pretty cool. How it works:
1. In variables.less, you add a few lines of comments before a variable declaration, indicating the "group" (for the admin UI), field name, field type (current options: color, text, pixels) and optional default value.
2. These comments are interpreted by the system to build the "CSS Variables" admin page in the admin UI under "Appearance"
3. Changing the settings in the admin UI produce new compiled CSS served to site visitors.

How part 3 works is a little weird, as we're not having less actually write a new file to the theme's CSS directory, as there are filesystem permission assumptions there we didn't want to make. Instead we take advantage of template overrides, transient settings and a custom post type to generate the compiled CSS on-the-fly (but still cacheable by browsers).