Added InAppRequest

SwiftyStoreKit/InAppProductQueryRequest.swift

@@ -26,11 +26,13 @@ import StoreKit
 
 typealias InAppProductRequestCallback = (RetrieveResults) -> Void
 
-protocol InAppProductRequest: class {
+public protocol InAppRequest: class {
     func start()
     func cancel()
 }
 
+protocol InAppProductRequest: InAppRequest { }
+
 class InAppProductQueryRequest: NSObject, InAppProductRequest, SKProductsRequestDelegate {
 
     private let callback: InAppProductRequestCallback