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.
I would add an explicit check, and if we fail the second check, bail early and return `nil` (or use your favorite variant of `UIColor *returnColor = nil;`).