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
edx.org: syncing up capitalization of register/login submit button copy
Merge pull request #486 from edx/talbs/mktg-vizsync

edx.org Login, Register, and iframe-based button visual sync