Cron module upgrades

  - added cron_file attribute: if specified, the file with appropriate
job is created in /etc/cron.d directory. Also, you can store multiple
jobs in one file. state='absent' attribute is handled in the following
way in this case: if after the deletion of the job from the file specified
by cron_file variable the file is empty, the file is deleted, otherwise
not.
  - fixed the behaviour, when the backupfile is saved forever in /tmp
folder, even if the backup= atribute is not set (os.unlink() is called if
backup is not True).
  - added some comments to the unobvious places