此方法用于通过调整不透明度的变化至指定目标来实现所有匹配元素的淡入效果，并在动画完成后可选地触发一个回调函数
实例:
<html lang='zh-cn'>
<head>
<title>Insert you title</title>
<meta http-equiv='description' content='this is my page'>
<meta http-equiv='keywords' content='keyword1,keyword2,keyword3'>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type='text/javascript' src='./js/jquery-1.12.1.min.js'></script>
<style type="text/css">
*{margin:0;padding:0;}
html{font:400 15px/1.2em 'Courier New';color:#666;750px;margin:25px auto;}
#show{250px;height:400px;background:red;display:none;color:#FFF;line-height:400px;text-indent:8px;padding:10px 30px;margin:70px;text-align:center;}
button{padding:8px;border:0;outline:none;background:#00E92C;border-radius:2px;}
.wrapAll{color:#FF96EC;}
</style>
<script type='text/javascript'>
$(function(){
$('button').click(function(){
$('#show').fadeTo(1000,.3,'linear',function(){
/*
实现将透明度调整至指定目标
*/
console.log('fade animate already Finished...');
});
});
});
</script>
</head>
<body>
<div id='demo'>
<button type="button">button</button>
<div id='show'>Test text</div>
</div>
</body>
</html>