CPColor hexToRGB doesn't support CSS-style colors with 3 chars

In CSS, when creating a color from a hex value, it is commonplace to abbreviate colors which have the same character for each if the channels using only three characters, i.e., one character for each channel. cccccc becomes ccc, aabbcc becomes abc, and so on. This is currently not supported by CPColor.

I have a patch already and will send the commit to the dev-list once I've signed the contributor's agreement.