[Feature Request] Support Cron Scheduling Syntax

[theycallmemac]

For those who are accustomed to writing the cron schedule syntax, the Odin Engine should be able to accept regular cron notation as a means by which to schedule jobs.

This is actually quite a simple thing to do, but issue #33 takes precedence.

[theycallmemac]

This probably shouldn't be touched until #33 is closed. Makes it simpler, less rewrites needed.