Added ResponseHandler property and default implementation function

[darrelmiller]

This ResponseHandler function will be called from the operation methods to process the HTTP response message. During construction of the request, the handler can be replaced with an alternative one. We should also consider the possibility of wanting to globally replace the default handler.

        public static Task<object> DefaultResponseHandler(HttpResponseMessage response) { return null; }
        public Func<HttpResponseMessage,Task<object>> ResponseHandler {get;set;}  = DefaultResponseHandler;

Once we have models in place, the object type in these signatures should be replaced by the model type for the request builder.