Added the *zoom: 1; rule to apply the Nicolas Gallagher micro-clearfix

Neither IE 6 nor 7 support `display: table;` nor pseudo-elements, so the clearfix solution currently in style.css won't work for those browsers.  The fix, in [Nicolas Gallagher's micro clearfix post](http://nicolasgallagher.com/micro-clearfix-hack/), is to apply `*zoom: 1;` that will trigger hasLayout on those browsers.

Since WordPress' stated browser support continues to be IE7+ (this has been reiterated to me by Nacin during the 3.7/3.8 cycle), the stylesheet for _s should support those browsers out of the box as well.