Remove Bluebird dependency

[jwerre]

We should be able to remove Bluebird as a dependency in favor of standard ES6 Promises.

[jwerre]

Doing so would change the amount of versions of Node we support.
#15 (comment)

I'm just going to copy & paste my comment from the old repo:

It may be worth considering adding a feature where you could plug in your own Promise library (oauthjs/node-oauth2-server#621 (comment)). Since Bluebird has been such an integral part of this project many legacy applications may be depending on the extended Promise functionality that it offers. It would be much easier to plug in Bluebird in many cases that to rewrite all that code. Furthermore, if you're using a feature like Cancelations you might find yourself is a situation where upgrading is untenable. One module that did this well is Mongoose—here's what that looks like.

That said, let's have a discussion on the minimum version of Node.js we should support.

[jankapunkt]

I already mentioned in the other issue that we should stick with what is officially supported by nodejs and I think in that case there is no need to have a shim. I think the last supported node is 12.x right?