Remove Bluebird Dependency

[jwerre]

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

[jwerre]

It may be worth considering adding a feature where you could plug in your own Promise library (https://github.com/oauthjs/node-oauth2-server/issues/621#issuecomment-880112523). 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.

[duyleomessi]

Is this feature done?

[jwerre]

Doesn't look like it: https://github.com/oauthjs/node-oauth2-server/blob/015416563fcd5f0dd58e562aabd1b117d4bfa361/lib/response-types/token-response-type.js#L9