Implement node-style callbacks

I do not know if this has been brought up before, so I'm hoping for the best now.
The only issue I could find was #242.

It would be nice if the node-style callbacks was supported. To me, it looks like it is not that complicated to achieve.

I've probably forgotten some of the public APIs, but this is a start. I'll update the pull request if there is positive response to this.

Maybe it needed some tests too?