CMNavBarNotificationView 介绍
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:
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Animation and layout similar to iOS 6 notifications
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Provides two UILabels and an UIImageView for customization
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Simple API
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Enqueues multiple notifications and shows them with a default duration of 2 seconds.
CMNavBarNotificationView 完全基于 MPNotificationView 进行扩展。