Added ability to use FlxColors for color attributes

For: https://github.com/HaxeFlixel/flixel-ui/issues/43

Uses reflection (sorry larsiusprime, haven't learned how to use haXe macros yet) to attempt resolution of a color first. If not, it'll resort to trying hex values using the existing methods. 
Also, it only handles upper-case colors. Would be easy to have it handle lower-case as well if desired.