Move redundant absolute positioning to shared styles and handle zero border width better

Hi,
it seems like the absolute positioning is the same for :after and :before in all cases, so there is no need to put it twice in the CSS. I moved it to the shared styles section (baseCSS) to save the user a redundant line. :)
Then there's the special case where you end up with 2 :after rules and no :before rule for a border width of zero, which I simplified into a single :after rule.
I also updated the unittests to reflect the changes.

What do you think?

Cheers,
bra1n