code: function(method, url, timestamp) {
          var data = [method, url, this.options.publicKey, timestamp].join('|'),
              signature = crypto.sha256(this.options.privateKey, data);
  
          return signature;
      }
docs: Generate a signature for the given parameters
  
  @param  {String} method
  @param  {String} url
  @param  {String} timestamp
  @return {String}