Big refactor to work in a multi-tenant style

Hi!

Thanks for doing the initial work on this.

I've added some functionality to redirect to Kounta URLs and get access tokens so the gem can be used in an authentication flow. README updated too, hopefully it's pretty clear. That said, it's a pretty big change from the current structure so I'm not sure if it ties into how you saw the gem working - interested in thoughts.

Am working on updating the tests at the moment.