wdn-clear class added to clear wdn-cols

By adding a class to the grid-set to clear wdn-cols.  This will clear each additional "row" of wdn-cols. 

example of class naming: "wdn-grid-set-halves wdn-clear" 

![screen shot 2013-10-14 at 2 46 00 pm](https://f.cloud.github.com/assets/3171887/1328823/9568d15c-350a-11e3-969e-616aff0be7e0.png)