Enable plugin for use with Federate

Hey @runemadsen, Marcel and I have gone over this changeset. It's working well with our app. Let me know what you think--thanks!

- allow redirect_uri to be set via Devise.capturable_redirect_uri
- allow endpoint to send authorization code to be overridden
- add documentation for overriding Javascript
- couple of things I had to do when merging in upstream changes