Ability to center top-banner ads

Currently we line up the top banner ad with the left hand side of the content

This gives us less room to play with, i.e. we can't load in larger ads at smaller breakpoints -> limiting what inventory we can sell

I've styled up the top banner so it's centre-aligned, gives us more room to play with. Need to weight up the aesthetics with the commercial benefit

It's behind a switch

![screen shot 2014-09-17 at 17 56 41](https://cloud.githubusercontent.com/assets/74132/4307746/1248ec9e-3e8c-11e4-9260-d6e460bae265.png)