Adds Download Button for Exercise

I created this branch to add the download button for the HTML/CSS Exercise. Feel free to move the button or add paragraph text beneath it. 

![screen shot 2013-06-01 at 4 12 22 pm](https://f.cloud.github.com/assets/631369/595590/c739ed42-cb10-11e2-9ad5-b0feb5b38cc4.png)

cc/ @evagabriela @pamelafox