Change home page Featured sets design.

Closes #505 

![home-page-features](https://cloud.githubusercontent.com/assets/96776/3622062/1f39a484-0e26-11e4-9f85-badde1f11f6d.png)

![home-pge-responsive](https://cloud.githubusercontent.com/assets/96776/3622064/224d9dd8-0e26-11e4-9d2d-bfab8abd4c5c.gif)

@jchristo4 the explicit blank alt and title attributes are from @jvine's spec in https://github.com/sul-dlss/SearchWorks/issues/505#issuecomment-49371303