Add code to create Basic authorization for headers in request to oauth.io /oauth2/token URL

[nethsix]

In src/oauth2/index.coffee, under token() method, there is a hard-coded basic auth token (this won't work anywhere), we need to replace this with the code that can create Basic authorization for the client using its clientId, and clientSecret returned from postauthorize.

[nethsix]

@OlehMO postauthorize response is something like:

{"status":"success","data":{"client_id":"{ client_id }","client_secret":"{
client_secret }","provider_id":"{ provider id provided by OAuth.io, which we store in config.js/config.local.js }","name":"Default app","user_id":"{ user_id (integer) }","app":"{ OAuth.io app id }", "id":"{ client_id }","redirect_uri":"https://oauth.io/auth"}}`

[nethsix]

@OlehMO The code I'm talking about is in another branch. Sorry I did not add in the branch name.

https://github.com/oauth-io/oauthio-server-node/blob/add-logging-index-js/src/oauth2/index.coffee#L101