code: function Credit(coin, spent) {
    if (!(this instanceof Credit))
      return new Credit(coin, spent);
  
    this.coin = coin || new Coin();
    this.spent = spent || false;
    this.own = false;
  }
docs: Credit (wrapped coin)
  @alias module:wallet.Credit
  @constructor
  @param {Coin} coin
  @param {Boolean?} spent
  @property {Coin} coin
  @property {Boolean} spent