PPFifoQueue: First in first out queue. See Unit test for more information.
PPAlertController: Bridge the compatiable gap between iOS7 and iOS8. If you are using AlertView/Action in iOS8, you are very likely to crash the app. However, you could not present the UIAlertController in iOS7. As a result, PPAlertController manager the compatiablity between 2 platform. It adapts UIAlertController API to configure AlertView/Actionsheet.