动态加入一段script的引用

 <script type="text/javascript">    var _gaq = _gaq || [];  _gaq.push(['_setAccount', 'UA-23029568-1']);  _gaq.push(['_trackPageview']);   (function() {    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);  })(); </script>