Include 'transparent' and update the CSSS submodule URL

'transparent' is included in the CSS3 spec (see http://www.w3.org/TR/css3-color/#transparent), and one.color( 'transparent' ) didn't seem to be working. I'd love for it to include support!

Also updates the CSSS submodule URL. It was pointing to https://github.com/LeaVerou/CSSS, while it seems the repo name has been changed to https://github.com/LeaVerou/csss (lowercased). Recursive checkout was failing before this was changed.