Flag cron schedules less than 15 minutes
Fixes #84
Correct crons to be measured in seconds not minutes

Add static class test
Add more complicated/tricky arithmetic
Always use eval for computing expression, as array_product yielded false positive
Add support for checking for cron_schedules in closures
Merge pull request #110 from WordPress-Coding-Standards/issue/84

Flag cron schedules less than 15 minutes