Bug 11577: Add an automatic renewal cronjob

Bug 11577: Add an automatic renewal cronjob

This patch adds a new cron script automatic_renewals.pl and a new
entry in crontab.example.

To test:

1) You need a few issues, some with automatic renewal and some without.
2) Confirm that each time you run misc/cronjobs/automatic_renewals.pl
   those issues are renewed that meet all of the following criteria:
   - automatic renewal has been scheduled either by issuing rule or by
     checkbox on the checkout page
   - the number of allowed renewals isn't exceeded
   - renewal isn't premature (No renewal before)
3) Confirm that all other issues are not affected.

Sponsored-by: Hochschule für Gesundheit (hsg), Germany
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>