Updated for modern times

This will send POST requests now, instead of GET, because it seems that's what Amazon prefers.  Also, requests are signed with HMAC-SHA-256 instead of HMAC-SHA-1.  This merges bpuklich's throttling code, adds ListOrders and ListOrderItems API methods, and adds a Get<ServiceName>ServiceStatus method for each Service (as defined by Amazon, e.g. Orders, Fulfillment) that supports the GetServiceStatus API call.