Add verifySubscriptions method to check all subscriptions in a group at once

[bizz84]

Related issue: #194

This PR adds a new method:

/**
 *  Verify the validity of a set of subscriptions in a receipt.
 *
 *  This method extracts all transactions mathing the given productIds and sorts them by date in descending order. It then compares the first transaction expiry date against the receipt date, to determine its validity.
 *  - Note: You can use this method to check the validity of (mutually exclusive) subscriptions in a subscription group.
 *  - Remark: The type parameter determines how the expiration dates are calculated for all subscriptions. Make sure all productIds match the specified subscription type to avoid incorrect results.
 *  - Parameter type: .autoRenewable or .nonRenewing.
 *  - Parameter productIds: The product ids of the subscriptions to verify.
 *  - Parameter receipt: The receipt to use for looking up the subscriptions
 *  - Parameter validUntil: Date to check against the expiry date of the subscriptions. This is only used if a date is not found in the receipt.
 *  - return: Either .notPurchased or .purchased / .expired with the expiry date found in the receipt.
 */
public class func verifySubscriptions(ofType type: SubscriptionType = .autoRenewable, productIds: Set<String>, inReceipt receipt: ReceiptInfo, validUntil date: Date = Date()) -> VerifySubscriptionResult

This makes it easy to find the most recent expiration date for a set of subscriptions, and is well suited to check validity within a subscription group.

Example log with output of verifySubscriptions:

• Add verifySubscriptions method to check all subscriptions in a group at once
• Rename verifySubscription(type:productId:inReceipt:validUntil:) to verifySubscription(ofType:productId:inReceipt:validUntil:)