Add IE8 compatibility

Adds html5.js to allow IE8 to recognize html5 and thus fix major layout problems. 
Adds respond.js to enable media queries in IE8.
Adds option under Global>Advanced to enable IE8 compatibility.

Attached screenshots show a blog page with Magazine layout without the option (before) and with the option enabled and indicated CSS added in child theme(after). 
<h2>Before</h2>
![before](https://f.cloud.github.com/assets/1617798/1113422/be723062-19fc-11e3-9495-cdfd9c5aeef5.png)
<h2>After</h2>
![after](https://f.cloud.github.com/assets/1617798/1113421/be668c12-19fc-11e3-9efa-08891bce5981.png)