Working Bearer option with tests.

Hi,

I've created the new provider as you recommend in this PR https://github.com/Vestorly/torii/pull/107

> A bearer provider is totally welcome, but I suggest you build it by subclassing the OAuth2 Code provider. If you do that the only thing that should change is the required URL params and response type.

> And a new provider should be tested.

It handles the open method slightly differently as it needs to return the token now, not a code - but apart from that hope I've done everything as you suggested.

It's working fine in my app so hope it might be useful to others trying to implement this flow.

Thanks again for your patience & advice :)