Inheriting box sizing v2

New patch for inheriting box sizing (replaces the now closed #630). Removed my previous "unclean" patch and replaced with this one which only includes the actual changes, no historical data.

This patch is based on the same idea as #609 but also has a patch for SASS to make the two stylesheets consistent.

## Background:
This patch improves box-sizing in accordance with http://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/ and http://blog.teamtreehouse.com/box-sizing-secret-simple-css-layouts#comment-50223 to allow for easier changes and alterations of the property in components and plugins that rely on other property settings.

Works the same as before, but improves functionality marginally and is considered the new best-practice approach.