Add executing user option to template and other mods.

David, thank you for the work you did on the opscode cron recipe. I have extended it to support the 'user' field present in system cron files (as opposed to 'crontab -e' files). The patch includes some additional lines in the template file, and removal of tabs and CR characters which were confusing things.

Finally, it includes  'if' node['cron']  ... end  to stop the absence of a 'cron' attribute in the node data from causing ruby to crash : otherwise I have to have 'cron' => {} .