Add OrderItem.cs

[mikelarkin]

API v2 has new URI to retrieve just order item information. We need to support this.

http://www.fetchapp.com/pages/help-api2#order-items

• id (integer)
• sku (string)
• order_id (string)
• product_name (string)
• price (float)
• download_count (integer)
• custom_1 (string)
• custom_2 (string)
• custom_3 (string)
• created_at (datetime)
• files_uri (string)
• downloads_uri (string)

[ranjeetkarn]

Thanks Mike for giving the info about fields