Feature/rename product to payment with quantity

[bizz84]

Fix for #200.

This is an API breaking change

Product renamed to Purchase (+ added quantity):

public struct Purchase {
    public let productId: String
    public let quantity: Int
    public let transaction: PaymentTransaction
    public let needsFinishTransaction: Bool
}

PurchaseResult

From this:

public enum PurchaseResult {
    case success(product: Product)
    case error(error: SKError)
}

to this:

public enum PurchaseResult {
    case success(purchase: Purchase)
    case error(error: SKError)
}

RestoreResults

From this:

public struct RestoreResults {
    public let restoredProducts: [Product]
    public let restoreFailedProducts: [(SKError, String?)]
}

to this:

public struct RestoreResults {
    public let restoredPurchases: [Purchase]
    public let restoreFailedPurchases: [(SKError, String?)]
}