Improve Cron class

This adds full documentation to the Cron class (implementation details etc, because IMHO it's a great way to learn and should be documented as much as possible) and improves the GCD algorithm to calculate the sleep time.
* Now using *filter()* plus lambda expr instead of *ok* version
* In python, *0 === x mod 1* for all integers *x*, so it makes no sense to check if any interval *=== 0 mod 1* (always true), plus it makes no sense to initialize *sleep* to 60, because it will be set to 1 anyway