Rename sass folder to scss

Probably more correct call it sass since that’s the used syntax.
Updated keywords for Bower

SCSS added and mobile-first modified.
Updated .gitignore and readme

Updated this files according the name change of SASS folder to SCSS.
Border-box sizing model to all elements

Border-box sizing model even to :after and :before elements
Merge pull request #30 from GabrielGil/master

Changed name of SASS folder to SCSS