Groundwork to enable 3rd party receipt validation

• Define ReceiptValidator protocol and default AppleReceiptValidator implementation
• This is passed to SwiftyStoreKit.verifyReceipt() as follows (API breaking change):

let appleValidator = AppleReceiptValidator(service: .production)
SwiftyStoreKit.verifyReceipt(using: appleValidator, password: "your-shared-secret") { result in
     // handle result
}     

See #126 for additional reference.