Adding "mkdir -p" to the log dir.

Makes sure the log dir is created before accessing it. Prevents some errors on
first run.
multi_log.php: Fixing uninitialized arrays if no server was selected.
Updated README.md.

* Added a link to the live demo/simulation.

* Changed nested list indentation to 4 spaces (see
  http://bitbucket.org/site/master/issue/7748 for details).