Switch to use promises

[morungos]

I never documented this at the time, but it's a big change that would be simple and useful. We should basically use promises rather than callbacks. It'll improve the error handling no end, and remove the drift right that happens in the longer code. In one sense, this is just a nice to have, but the code is actually a bit hard to maintain right now because of it, especially with respect to error handling.

[Remimstr]

This issue was moved to lstein/Heliotrope#17