Canvas钟表控件，使用方法如下：
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<script type="text/javascript" src="Clock.js"></script>
</head>
<body>
<canvas width='500' height='500' id='clock'></canvas>
</body>
<script>
initClock('clock');
</script>
</html>
