Move away from cron-utils #14

Cron-utils does not follow the spirit of Wisp: keep it small and efficient. Moreover, cron-utils has recurring security issues due to poor design choices and especially relying to javax.el.

https://github.com/frode-carlsen/cron/tree/master/java8 seem a good candidate: everything is contained is one class of 650 lines, without any dependencies. Though it does not seem to be available on Maven central.