public MailChimpServiceResponse List(CampaignFilter filter, int start = 0, int limit = 25, string sortField = "", string sortDirection = "")
 {
     return WaitForServiceResponse(ListAsync(filter, start, limit, sortField, sortDirection));
 }
        /// <summary>
        /// http://apidocs.mailchimp.com/api/2.0/campaigns/list.php
        /// </summary>
        /// <returns></returns>
        public Task<MailChimpServiceResponse> ListAsync(CampaignFilter filter, int start = 0, int limit = 25, string sortField = "", string sortDirection = "")
        {
            var url = Urls.Campaign + "list.json";

            var request = new {
                filters = filter,
                start,
                limit,
                sort_field = sortField,
                sort_dir = sortDirection
            };

            return Execute(url, request);
        }