edx.org: adds in variables/mixins to support edx.org visual changes
edx.org: applies edx.org visual changes (with overrides/new mixins)
edx.org: handles letter-spacing on buttons
edx.org: revises iframe-based register/status buttons to match revised styles
edx.org: revising base link color variable to use darker value for accessibility
edx.org: abstracting new edx.org  button extends and colors with other themes in mind
Merge remote-tracking branch 'origin/release' into rc/2013-07-30