Add some evalex-related switches to indico_shell

- To globally enable evalx, use --enable-evalex. This gives you a python
  shell when a traceback is displayed and also enables a python shell
  via http://your.hostname/console
- If your webserver is not listening on localhost only, it is HIGHLY
  recommended to use --evalex-from to restrict access to trusted ips
  such as the one of your machine. Remember, anyone with access can
  executed arbitrary Python code on your machine which allows him to do
  anything the user running indico_shell can do!