默认已经注册TRC20的合约ABI，底层封装了自动根据合约地址拉取合约ABI的机制。一般来说不需要手动注册ABI，有些合约ABI拉取不到的时候需要手动注册。
注册ABI的方式与以太坊一致，例如：
// USDT合约地址：TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t
let abi = `[{"constant":true,"inputs":[{"name":"who","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"}]`
// 注册 balanceOf 方法
exchange.IO("abi", "TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t", abi)