Extra color functions about closure-stylesheets HOT 6 OPEN

Could you please upload this to codereview.appspot.com? Things are not configured to
ingest contributions yet, but I will try my best.

In general, it would be easiest to follow the instructions on http://code.google.com/p/closure-compiler/wiki/Contributors,
though http://code.google.com/p/closure-library/wiki/Contributors has more detailed
instructions for using codereview.appspot.com.

OK. I'll clean a little the code (it's a little dirty and I've idea to do it better)
and when it's good I'll upload on codereview. I think it will be available in the next
days.

Here is the patch uploaded to codereview : http://codereview.appspot.com/5468048/

I've update many things from the first patch (linked to this issue). Essentially by
adding some HSL functions (Luminance is used to have a lighter / darker color).
Some tests has been written and the issue contains a little test case to see the usage
of those functions.

Hope this can be usefull