code: function(oauth2_server_id, res, next_url, state) {
      var sconfig = this.config.servers[oauth2_server_id];
      var cconfig = this.config.client;
      var data = {
        client_id: sconfig.client_id,
        redirect_uri: cconfig.redirect_uri,
        response_type: 'code',
        state: this.serializer.stringify([oauth2_server_id, next_url, state || null])
      };
      var url = sconfig.server_authorize_endpoint +'?'+ querystring.stringify(data);
      redirect(res, url);
    }
docs: Redirects the user to OAuth2 server for authentication.
  
  Arguments:
  - oauth2_server_id: OAuth2 server identification (ex: "facebook.com").
  - res
  - next_url: an url to redirect to once the process is complete.
  - state: optional, a hash containing info you want to retrieve at the end
  of the process.