Refactor into story-hero class for bg image, and reflow text

See new flow of text below.

![crossfit tonbridge gocardless customer stories gocardless](https://cloud.githubusercontent.com/assets/2903904/4574479/c3d5329e-4f9c-11e4-9c90-08e3118a95c5.png)
![the uk s 1 for online direct debit gocardless](https://cloud.githubusercontent.com/assets/2903904/4574480/c6e6d4ba-4f9c-11e4-8428-03139c25969d.png)