#399 cron job extension in system/core

[irmtfan]

Now many modules and scripts need some kind of cron jobs.
eg: Xortify, Xhelp to fetch emails, ... Also can be used for any scripts like PayPal callback.
IMO it is good to unify all these functionality in one extension in the core.

Then this extension can be called by any module/function need it.
related topic in xoops.org at roadmap 2.6 discussion:
http://xoops.org/modules/newbb/viewtopic.php?post_id=348199#forumpost348199