implement missing shade() function

[skierpage]

http://www.lesscss.org/#reference has

shade(@color, 10%);     // return a color mixed 10% with black

But lessc.inc.php does not implement this. There are several other functions in the less reference missing in lessphp: greyscale, tint, hsv*, etc. I'll file a separate bug that the docs should mention the incompatibilities.