added missing reset method

This method has been implemented in the latest version of Postbox.

knockout.postbox/knockout-postbox.d.ts

@@ -10,6 +10,7 @@ interface KnockoutPostBox {
     publish<T>(topic: string, value?: T): void;
     defaultComparer<T>(newValue: T, oldValue: T): boolean;
     serializer: (object: any) => string;
+    reset(): void;
 }
 
 interface KnockoutObservable<T> {