4.0.0 - CSS Grid and Styleguides

This release adds official support for CSS Grid to Responsive Foundation, along with dynamically generated styleguide support.

• Adds CSS Grid support to the grid, with an automatic fallback to floats
  for unsupported browsers.
• Adds support for color stops to the linear gradient mixin. New syntax:
  @include linear-gradient(to right, #E47D7D 0%, #C195D3 50%, #4FB4E8 100%);
• Ensures BU Hub required icons are always output, even if you turn icons off.
• Adds a skip link mixin and moves styles to the accessibility partial.
• Fixes #208
• Adds support for KSS documentation and officially switches us from SassDoc