Update clearfix to use overflow: auto

Clearing floats with overflow: auto works since IE7+.

Simple test: http://jsbin.com/odirez

Tested with Sauce App’s VMs and works on:

IE: 7, 8, 9, 10
Firefox: 3+
Safari: 5+
iOS: 4.3+
Opera: 9+
Chrome

Inspired by Nicole Sullivan’s wonderful article: http://www.stubbornella.org/content/2009/07/23/overflow-a-secret-benefit/