Fix for Incompatible units: 'rem' and 'px'

When setting a size for the switch heights, the defaults are set in rem and comparing it to px is causing a syntax error. So I switched it to use the rem-calc() mixin to resolve the error.
Add What You Get page and Stylings in Docs.SCSS for styling
Add content to Whatyouget html & fix Sass page
Merge pull request #5391 from jameswlane/patch-2

Fix for Incompatible units: 'rem' and 'px'
Merge branch 'master' of https://github.com/zurb/foundation
small.error and span.error seem to be interchangeable for holding Abide error messages. Added a missing `.error span.error` selector.