Updated Reddit to use authorized_response (and streamlined implementation)

Further to https://github.com/gae-init/gae-init-auth/pull/40 improved authentication using Reddit as reported in https://github.com/gae-init/gae-init-auth/issues/34, removing `@authorized_handler is deprecated in favor for authorized_response` warning and streamlining the Reddit implementation a bit further. Some things seem to be handled by Flask-OAuthlib / oauthlib already, while the Reddit specific override is renamed and put more in sync with the library code.