PROPOSAL - center background

The background image is left aligned and looks odd when the page is wide and the content is center aligned.

BEFORE

![screen_shot_2013-10-21_at_8 38 08_am](https://f.cloud.github.com/assets/486512/1369177/17d01b58-39db-11e3-9d98-e91b93df807e.jpg)

AFTER

![screen_shot_2013-10-21_at_8 43 39_am](https://f.cloud.github.com/assets/486512/1369178/1bf5e69a-39db-11e3-9d23-fc05d2dab6db.jpg)

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- [ ] @HelenRoss 
- [ ] @antfrew 
- [ ] kate
- [ ] dev