Use bluebird promises to improve performance

[Stateford]

Built in node.js Promises are 4x slower than blue bird, It's better to bring in the package bluebird and add the line js const Promise = require('bluebird');

I can do a PR if you think it's warranted, but it's probably a good idea overall to do it.

[claviska]

I was wondering about Bluebird, but wasn't aware it was actually faster. Good to know. It's already being used by some of Postleaf's deps, so I don't mind adding it to package.json. A PR would be great.

We could probably use Promise.mapSeries to address #5 as well.