Tidying up OAuth2Session

[ib-lundgren]

The whole _client.client indirection is pretty awkward and confuses me all the time. It might be time to get rid of the requests_oauthlib.OAuth2 _client by copying in it's functionality directly into the OAuth2Session.

That together with new and coming changes to oauthlibs OAuth 2 client (e.g. full prepare_x_request methods and state management) should make the OAuth2Session a bit clearer as well as leaner.

example of _client being an annoying indirection #153