Support for multiple oauth use cases

Hello,

This change allows developers to create single heroku app to exchange github code for access token for multiple domains.

This may be useful if you are developing your app and you client_id/client_secret is bound to localhost domain, while in production you are using different combination of client_id/client_secret for production domain.

The PR also updates readme file to use official [heroku CLI](https://devcenter.heroku.com/articles/heroku-command), instead of `cake`.

I know it's a lot of breaking changes, and this project is not actively supported anymore. But hey, if someone finds this useful, I'll be happy :).