Tweaks to padding/border/content-box attribute

Apparently older versions of Firefox and current versions of webkit
support a sort of `alias` for padding/border/content-box which is
simplified without the -box.

This means that padding-box == padding, and content-box == content

For more info:
https://developer.mozilla.org/en-US/docs/Web/CSS/background-origin
Merge pull request #31 from amadeus/fix-padding-attr

Tweaks to padding/border/content-box attribute