Remove use of promise module

[ryzokuken]

We seem to be using the old promise npm module https://www.npmjs.com/package/promise which made a lot of sense back when this was initially written, but Node.js natively supports promises now. Switching to native promises should have multiple benefits, including perf gains and better stack traces.

Blocked by #4

/cc @edef1c @vachi @HasmikAtom