Swift 4.1 / Xcode 9.3 Support

• Swift 4.1 / Xcode 9.3 Support (#360)

Add support Ask To Buy

• Add a wrapper for simulatesAskToBuyInSandbox (#349)

Add support for downloading content hosted with Apple

• Add support for downloading content hosted with Apple (#343, related issue: #128)
• Add table of contents to README (#346)

Add `verifySubscriptions` method for subscription groups

• Add verifySubscriptions method to check all subscriptions in a group at once (#333, related issue: #194)
• Rename verifySubscription(type:productId:inReceipt:validUntil:) to verifySubscription(ofType:productId:inReceipt:validUntil:) (#333)
• Add video tutorials section in README (#328, #330, see #326)
• Update iOS Demo App (#327, see #147)

`completeTransactions` finishes failed transactions if `atomically: false`

• Add originalTransaction to PurchaseDetails (#323, fix for #302)
• Bug fix: completeTransactions finishes failed transactions if atomically: false (#322, related issues: #307, #288)

Remove `SKProduct.localizedIntroductoryPrice`

• Remove localizedIntroductoryPrice (#320, see #319, #318, #315)

Add `PaymentTransaction.transactionDate` and `SKProduct.localizedIntroductoryPrice`

• Add transactionDate to PaymentTransaction (#316, see #312).
• Add localizedIntroductoryPrice to SKProduct (#318).

Add `fetchReceipt` method + update `verifyReceipt` and `ReceiptValidator` protocol

• Add fetchReceipt method. Update verifyReceipt to use it (#278, related issues: #272, #223).
• Update fetchReceipt and ReceiptValidator to use receipt as Data rather than String. This is consistent with localReceiptData (#284, see #272).
• Remove password from ReceiptValidator protocol as this is specific to AppleReceiptValidator (#281, see #263). Note: This is an API breaking change.
• Unwrap receipt["receipt"]?["in_app"] in two steps (addresses casting problems) (#283, related issue #256).

Update to swiftlint 0.22.0

• Update to swiftlint 0.22.0 (#270, fix for #273)

Fix for concurrent retrieveProductsInfo calls

• ProductsInfoController: Keep track of multiple completion blocks for the same request (#259, fix for #250)