added ability to add item by OfferListingId

Hi! I think it will be cool to have ability to add items to cart by OfferListingId in additional to ASIN out of the box.
Also amazon says that using OfferListingId is preferred instead of ASIN when adding items to a cart.
http://docs.aws.amazon.com/AWSECommerceService/latest/DG/CartCreate.html#CartCreate-rp

So this is my simple implementation, if you don't mind :)