Reorganize `FlxU`

Move several methods to new locations (as per [the FlxMath planning spreadsheet](https://docs.google.com/spreadsheet/ccc?key=0Avkk09kHolxjdEJPaHJMeFJmZTV1Rkt4bGgzV01HcVE&usp=sharing#gid=0))

    FlxU.rotatePoint() -> FlxPoint.rotate()
    FlxU.getAngle()    -> FlxPoint.angleBetween()
    FlxU.getDistance() -> FlxPoint.distance()
    
    FlxU.abs()   -> FlxMath.abs()
    FlxU.floor() -> FlxMath.floor()
    FlxU.ceil()  -> FlxMath.ceil()
    FlxU.round() -> FlxMath.round()
    FlxU.min()   -> FlxMath.min()
    FlxU.max()   -> FlxMath.max()
    FlxU.bound() -> FlxMath.clamp()
    
    FlxU.getTicks() -> FlxG.getTicks()