Use original user IP instead of REMOTE_ADDR when running in proxy mode.

When Cherrypy runs behind a front-server (e.g. Apache) the user IP is not
represented by the header entry “REMOTE_ADDR” anymore. One should use
“X-Forwarded-For” or “HTTP-X-FORWARDED-FOR” instead.

see http://www.xinotes.org/notes/note/770/ for details.

(from redmine: issue id 1042, created on 2012-06-02 by root, closed on 2013-01-07)