Add support for authorization flow

[justasm]

This wrapper fulfills the first purpose of Signpost -- signing HTTP messages -- but not the second -- requesting tokens from the OAuth service provider.

Would it be appropriate to add support for the authorization flow (i.e. via an OkHttpOAuthProvider and OkHttpResponseAdapter), or is this outside the scope of this extension?

[pakerfeldt]

It's definitely within scope. I just haven't come around doing it yet. I'm happy to receive a PR if you want to pursue that.