Adding Eventbrite Provider
When you make requests to the Eventbrite API using oauth2 access_token you need to send an Authorization header field with value of Bearer otherwise you get error.

This is not needed for all the oauth stuff just the last call to the API to fetchUserDetails.

I am not sure if this will brake other requests to other providers or if they will just ignore this extra header?
When sending an request to get an Access Token to Eventbrite you also have to send 'grant_type with value of authorization_code'.

Not really sure how to document this?
This might be a better way of explaining in the README how to call Eventbrite
Added Eventbrite to the list of built-in identity providers
Added Eventbrite to the list of inbuilt providers
Merge pull request #57 from jamesmills/master

Adding Eventbrite