CAanimation动画 暂停，继续

暂停：

        let pausedTime = layer.convertTime(CACurrentMediaTime(), fromLayer: nil)
        layer.speed = 0.0
        layer.timeOffset = pausedTime

继续：

        let pausedTime = layer.timeOffset
        layer.speed = 1.0
        layer.timeOffset = 0.0
        layer.beginTime = 0.0
        let timeSincePause = layer.convertTime(CACurrentMediaTime(), fromLayer: nil) - pausedTime
        layer.beginTime = timeSincePause