Datetime driver

 * Replaced `Cron` driver with `DateTime` driver
 * Reworked verbiage on driver documentation to better communicate their purpose
 * `DateTime` driver has these added methods: `quarterly()`, `weekly()`, `week()`, `weekOfYear()`, `everyOddWeek()`, `everyEvenWeek()`

This this pull request includes breaking changes, I'll leave this open for 24-48 hours for comments/feedback before merging it in.
Can we keep this please? The point of this is that locally people can have their own `phpunit.xml` when they are contributing, and then only the dist version get published.