Added FlxColorUtils

- New class `FlxColorUtils` containing over 30 new colors and the color
constants previously located in FlxG as well as color-related functions
previously located in FlxU added
- For the sake of backwards compatiblity, FlxU and FlxG still contain
the functions / variables, but they are just references to FlxColorUtils
and are documented accordingly
- The new colors have short descriptions from wikipedia pages