sizeClass

新增了Size Classes特性，它是对当前所有iOS设备尺寸的一个抽象，
iPhone4S,iPhone5/5s,iPhone6
竖屏：(w:Compact h:Regular)
横屏：(w:Compact h:Compact)
iPhone6 Plus
竖屏：(w:Compact h:Regular)
横屏：(w:Regular h:Compact)
iPad
竖屏：(w:Regular h:Regular)
横屏：(w:Regular h:Regular)

http://www.cocoachina.com/ios/20141217/10669.html

http://www.cocoachina.com/ios/20140915/9623.html