an image slider component with Vue
共有三级节点，最外层的节点固定，是展示给用户看到的部分；
There are three levels of nodes. The outmost node which is show to the customers is fixed.
第二层是包含图片组的容器；
The second level node is the container of pictures.
第三层是图片组，这里需要复制一份完全相同的轮播图片组。
The innermost node are the pictures. You need to copy one group of pictures here.
图片逐步向左移动，当demo中的四张图片全部移出到页面左侧时，立刻把图片恢复到初始化的位置，循环往复。
The container moves from left to right. In this demo, we should immediately restore the container to the initialized position when the first four pictures are all moved to the left side of the page. And then, continue the loop.
鼠标移入时重新执行动画，移出时再次开始执行动画。我们需要的效果是图片始终匀速移动，因此在 animate函数中执行时间需要在保持移动速度不变的情况下，动态设置。
When the mouse moves in the container, wo should stop the animation. Similary, when the mouse moves out of the container, we should restart the animation. In order to keep the speed constant, we should recalculate the time in the animate function when we restart the animation.