code: function PaymentACK(options) {
    if (!(this instanceof PaymentACK))
      return new PaymentACK(options);
  
    this.payment = new Payment();
    this.memo = null;
  
    if (options)
      this.fromOptions(options);
  }
docs: Represents a BIP70 payment ack.
  @alias module:bip70.PaymentACK
  @constructor
  @param {Object?} options
  @property {Payment} payment
  @property {String|null} memo