Update schedule plugin trigger spec

[pauldix]

The trigger spec for schedule plugins takes schedule:<some cron expression>. After working with it, I realize that it's probably better to have it be cron:<some cron expression> as @jacksonrnewhouse had it initially.

However, I'd also like to add every:<some human time duration> as an alternative syntax for specifying the schedule. Then it would run every time that duration of time passes. It should start on the interval (i.e. 1h would start at the top of the hour).