Remove name mangling which prevents subclassing OAuthClient.

I tried to subclass ``OAuthClient`` for a project* but found that the name-mangled instance attributes ``__access`` and ``__client`` made just rewriting from scratch easier. I've only been working with your guys' library for a day so I apologize if there's a specific reason for the dunder prefix, but inheritance is close to broken in this case.

\* *I wanted to get just the response object from ``_do_request`` so I could process HTTP requests based on their response code & body instead of catching exceptions thrown by ``OAuthClient``*