Improved box-sizing

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.