add cron tests

0 should be treated as a valid value for minute, hour and day_of_week
fields.
Fix #189

Only fall back to the default value of "*" for minute, hour and day_of_week
crontab fields if the passed value is undefined.
fixes ownership bug of files written with sudo
Merge remote-tracking branch 'ferki/cron-zero-values' into 0.42
updated build files