* Switch all coupon fields to _data
* Move coupon custom fields to _meta_data
* Shorten the 'yes' -> true checks
* Add a test for getting custom fields
* Rename individual use, free shipping, and exclude sale items to match their meta names closer.
* Sanitize emails when using set_email_restrictions
* Remove some unnecessary explict true checks
* Move ID into _data with the rest of the fields
* Fixed some spacing issues