Support for iOS 11 Featured IAPs

[MosheBerman]

iOS 11 adds a new mechanism for allowing users to purchase IAPs right in the App Store. There's a few small steps we need to take to support that.

• Adopt the new SKPaymentTransactionQueueDelegate method.
• Adopters need to add iTunes Artwork, so link the relevant Apple docs. Update README to point to relevant Apple documentation.

[MosheBerman]

Related API: https://developer.apple.com/documentation/storekit/skpaymenttransactionobserver/2877502-paymentqueue