Swift の enum型を for-inする方法

enum Num: String{
    case Zero = "0", One = "1", Two = "2", Three = "3", Four = "4", Five = "5", Six = "6", Seven = "7", Eight = "8", Nine = "9", Ten = "10", Eleven = "11", Twelve = "12"
}

このようなenumがあった場合にfor inさせるにはどのような方法がありますか？