图片拉伸

 bg = [bg resizableImageWithCapInsets:UIEdgeInsetsMake(10,10,10,10)];//默认是平铺 

 bg = [bg resizableImageWithCapInsets:UIEdgeInsetsMake(10,10,10,10) resizingMode:..];//平铺和拉伸

bg = [bg stretchableImageWithLeftCapWidth: topCapHeight:];

     把官方计算拷贝

     // 1 == width - leftCapWidth - right cap

     // 1 == height - topCapWidth - bottom cap

     // 只拉伸1*1的像素