hey @maxstrauch,
Thanks for this patch. Currently this library is designed to be backward comparable with node 6, and so async is not a supported approach in the source code. You can see that the tests failed for node 6, and if you look at the failure log, it will provide more detail.

I would accept a patch written in Promises form and with a test or two.

Thanks again

@monkpow I saw it too late that this lib want's to be backwards compatible and I didn't notice it when looking at the code since I only develop in ES6 or Typescript lately ;-)

I'll see what I can get done over the holidays ;-)

Fixed with Node 6 support here: #420