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 reword it thusly:
> A string, prefixed with “#”, followed by six lowercase hexadecimal digits. The alpha channel is ignored.