Constuct a new instance.

@param {Object} config Configuration.
@param {Object} config.web3 A `Web3` instance.
@param {Object} config.account Address of account to send transactions from.
@param {Number} config.gas Gas amount to use for calls.
function ContractFactory(config) {
      _classCallCheck(this, ContractFactory);

      this._config = config;
      this._web3 = config.web3;
      this._account = config.account;
      this._gas = config.gas;
    }