async version

[dkebler]

I can Sindre pify but since you have seen fit to create a promisified version of mqtt.js can that be done for this repo as well? Blanket promisification of a module has its issues. Thx.

[RangerMauve]

There has been talk about migrating MQTT.js to support promises out of the box, but I don't think any of the core maintainers have time for it at the moment. Contributions are of course welcome.