Add shadow effect for images

Hi, aigarsdz. Recently I have added shadow effect for images using `shadowbox.js`. I have tested on localhost. Here is the effect: [\[link\]](http://yuanyong.org/brume/demo-post/). The files I added include `shadowbox.css` and `shadowbox.js`. In the `default.html`, I initialized the `shadowbox.js` between `<head>`, and added the shadow in the `main.css`. It looks cool!