added minute interval allowance

Love this code...thank you...it has solved and consolidated many things for me.
The one "problem" I had was that running my "cron" every minute was going to be too much.  So, I looked into running it every 10 minutes and having the code detect what should have run within those 10 minutes.

I updated the isDue function to allow an interval of minutes to be checked
so that you would not need to run this function "once per minute" and
could pick up events within the designated interval.  also added tests
to demonstrate usage and check the boundaries.