流程和相关知识参考此文档：
http://www.cnblogs.com/XimuYouzi/archive/2016/04/17/5401749.html
支付凭证验证官方开发文档：
https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Introduction.html#//apple_ref/doc/uid/TP40010573-CH105-SW1
IAP官方文档：
https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/StoreKitGuide/PromotingIn-AppPurchases/PromotingIn-AppPurchases.html#//apple_ref/doc/uid/TP40008267-CH11-SW1