Crontab command as a user option

Some system does not allow the use of /usr/bin/crontab for resource policies reasons (for example on HPC clusters) Another home-made crontab command is available for such purpose on these clusters (same interface).

It would be nice to be able to set our own path to the crontab command.

CRONCMD would be the default in the CronTab class init, then the user can pass his own cmd.

proposed patch attached crontab.py.patch