Add rating and rating cout

[juampynr]

Amazon PAAPI started returning rating info for selected clients. When they activate such feature, it comes via these two properties in their GetItems responses.

Sample:

{
  "ItemsResult": {
    "Items": [
      {
        "Offers": {
          "Listings": [
            {
              "MerchantInfo": {
                "FeedbackCount": 385,
                "FeedbackRating": 4.68,
                "Id": "ATVPDKIKX0DER",
                "Name": "Amazon.com"
              }
           }
         ]
      }
    ]
  }
}

[spiegel-im-spiegel]

Many thanks!

[juampynr]

Just a comment to clarify something I found out later so I don't confuse anybody:

These two fields are merchant ratings, not ASIN ratings. ASIN ratings are available via EnableCustomerReviews().