Rewrite grid column mixin
Update README.md to reflect changes from ssgs
Fix wrong indentation in ssgs.scss
Update README.md

Changed demo link to the more recent CodePen version so I only have to update on at a time.
Added link to the CSS version.
Thrown in some more Markdown love (links'n'stuff).
Change .css snippet to the correct .scss snippet in README.md
Add .css version
Add license file
Change CodePen username
DRY update

Previously, three declarations would be included with every grid-col call, which is totally not necessary. Now, the first include extends `.grid-col` so there are no repeated declarations.
Remove CSS version and use silent classes