iOS 导航栏左右 BarButtonItem偏移

iOS7 之后，我们直接在导航栏添加barbuttonItem时候，会有一定偏移量，
比如：
self.navigationItem.leftBarButtonItem ＝ UIBarButtonItem(customView: btnLeft)
这样布局图片出屏幕，左右可多加一个空白按钮

解决方法：

leftBarButtonItems：
let leftItem = UIBarButtonItem(barButtonSystemItem: .FixedSpace, target: nil, action: nil)
leftItem.width = -20
self.navigationItem.leftBarButtonItems = [leftItem,UIBarButtonItem(customView: btnLeft)]

rightBarButtonItems：
let rightItem = UIBarButtonItem(barButtonSystemItem: .FixedSpace, target: nil, action: nil)
rightItem.width = -20
self.navigationItem.rightBarButtonItems = [rightItem,UIBarButtonItem(customView: btnRight)]