12栅格布局在IE8下不起作用。
下载并引用html5shiv.js和respond.js
参考：
1.http://nextflow.in.th/en/keep-your-responsive-web-design-against-the-legendary-internet-explorer-ie6ie7ie8/using-response-js-to-make-ie6ie7ie8-understand-your-responsive-design/
2.http://pengbos.com/blog/bootstrap-3-and-ie-8
3.http://www.extremecss.com/making-twitter-bootstrap-3-compatible-with-internet-explorer-7-and-below/