Improved performance of parallax and add hero banner to styleguide

The performance of the parallax has been improved:
Some tests scrolling the page.
Before:
![screen shot 2014-06-04 at 12 33 42](https://cloud.githubusercontent.com/assets/1152640/3172307/c3ea81c6-ebd3-11e3-9858-36a9dc978ba9.png)
After:
![screen shot 2014-06-04 at 12 34 23](https://cloud.githubusercontent.com/assets/1152640/3172314/d288b824-ebd3-11e3-92a9-8c8b50047d2d.png)

I've also improved the effect. Now, the whole image will be shown if you scroll up/down. Before, only a part of the image was shown.