Add refer http link on hack

tracim/tracim/config/__init__.py

 
     def after_init_config(self, conf):
         AuthConfigWrapper.wrap(conf)
-        #  Fix an tg2 strange thing: auth_backend is set in config, but instance
-        #  of AppConfig has None in auth_backend attr
-        self.auth_backend = conf['auth_backend']
+        #  Fix tg2 problem: https://groups.google.com/forum/#!topic/turbogears/oL_04O6eCQQ
+        self.auth_backend = conf.get('auth_backend')
         self.sa_auth = conf.get('sa_auth')