Styling Markdown (and other) previews with Twitter's Bootstrap

@zefhemel

I had to paste the minified CSS from the file straight into the document. I tried using a CDN to load it but it wasn't working so I gave up on that. I can give it another try if that is preferred though, I have an idea how I could try to fix it this time. Here's a preview:

![image](https://cloud.githubusercontent.com/assets/463132/2681426/a6b92b40-c18d-11e3-830e-cd9419e737c9.png)

I also added some padding, since otherwise it butted right up against the frame.