missing Promise.nodeify() added

I don't understand much of the second link, but the first one points to the correct function.

When I submitted my patch I hadn't noticed that the documentation reads: `If callback is not a function, simply returns promise`. When merging, please fix the return type. It's not `void`, but `Promise<U>`.

Source: https://github.com/kriskowal/q/wiki/API-Reference#promisenodeifycallback