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.
This should have vvdocumenter-style documentation specifying that it has a leading `#` and the alpha channel is ignored. (Or an alpha version should be added.)