Please support Implicit Grant flow

[genru]

Hi,

I found node-oauth2-server does not support Implicit Grant type described in RFC6749 section-4.2, which is a pretty simple authentication with only single request/response. And widely used in web app auth.

It would be great if node-oauth2-server could support implicit grant type.

[maxtruxa]

PR #271 implements Implicit Grant.

[mjsalinger]

That PR, #271 needs to be rebased and conflicts fixed. If anyone is interested in doing that, I'll merge it in once reviewed.

[clayrisser]

+1 for implicit grant support

[spawnrider]

+1 for it.