Add .list-benefits--s and .button--input 

* Creates a modifier of `.list-benefits`: `.list-benefits--s`
* Creates a button that sits alongside an input

![screen shot 2014-06-30 at 10 35 41](https://cloud.githubusercontent.com/assets/1579511/3428027/3c28a48a-003b-11e4-8cfd-61223cd8e451.png)