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.
Since `hexString` is built in the `else` clause, it is never built if we fall through the `-getRed:green:blue:alpha:` failure.