Added category method for returning a hex string from a UIColor in rgb color space

@ndonald2 This feature was part of a large dangling branch, but we need it in for our project, so I created a new branch just for this. There may be one or two more of these coming your way.
If this returns `NO`, is there a chance that trying to extract the color in a different way would work? Perhaps `getWhite:alpha:`? I’ve never seen the RGB version fail, but I suppose it’s possible.