Connect middleware to handle web based OAuth authorization

As discussed, here is a connect-middleware (session based) that handles the web based OAuth authorization. It mounts a route on the specified path. When a user is getting that path, they will be redirected to Dropbox for authorization of the app. On successful authorization, Dropbox will redirect the user back to the same path, where we store the token and secret in the session.

I'm not crazy about the method used, as I feel a consumer of the dropbox library might want greater freedom on how to handle the oauth token/secret, so if you have any suggestions, I'd be happy to rewrite it.