Callbacks

constructor(    getOfferings: GetOfferingsCallback? = null,     getBalances: GetBalancesCallback? = null,     getExchanges: GetExchangesCallback? = null,     getExchange: GetExchangeCallback? = null,     createExchange: CreateExchangeCallback? = null,     submitOrder: SubmitOrderCallback? = null,     submitClose: SubmitCloseCallback? = null)