Add alternative mobile layouts

- Layout A: equally distribute items horizontally
- Layout B: items 2 and 3 are horizontally aligned

Mobile layout variations should degrade gracefully into a vertical list*

Browser support:
- iOS 7+ (-webkit prefix needed)
- Chrome
- IE 11+
- Firefox 28+ (supports flex-wrap: wrap)

\* Firefox < 28 treats layout B as if it were a horizontal list.