Layers transition correctly

This time it handles scrolling. No need for scroll top.


![layers-with-scroll](https://cloud.githubusercontent.com/assets/814861/5318024/588c795a-7c94-11e4-955d-e726e5096061.gif)

I'm not sure that's all really necessary. I think a modifier class would actually do the trick here so we'd maybe pass the actual class through (instead of `styled`). That makes it way more flexible and would only require updating the styleguide page for the preloader so that it mentions this.