Three quarter item/slices

This adds the styling and Slice logic for the three-quarters item.

### Desktop
![screencapture-localhost-9000-alex](https://cloud.githubusercontent.com/assets/80089/4376607/0e8840ae-4347-11e4-9c35-a21a7db00794.jpg)

### Tablet
![google chrome](https://cloud.githubusercontent.com/assets/80089/4376614/16acc52a-4347-11e4-8524-a47e560193d4.png)

### Mobile
![screencapture-localhost-9000-alex 1](https://cloud.githubusercontent.com/assets/80089/4376618/1aa8e334-4347-11e4-9457-384775963b03.jpg)

/cc @robertberry @sndrs
these should probably go in `_slices.scss`?