promisify polyfill

promisify.js

// usage: promisify(instance.func.bind(instance))

function promisify(fun) {
    return function (...args) {
        return new Promise((resolve, reject) => {
            fun.apply(fun, [].concat(args, (err, res) => (err) ? reject(err) : resolve(res)))
        })
    }
}