CMNavBarNotificationView 实现了应用内的提醒效果，显示在导航条的顶部，如下图所示：
Features:
Animation and layout similar to iOS 6 notifications
Provides two UILabels and an UIImageView for customization
Simple API
Enqueues multiple notifications and shows them with a default duration of 2 seconds.
CMNavBarNotificationView 完全基于 MPNotificationView 进行扩展。
