Adds basic logging of stuck sustainers to watchdog.

 - Creates a new health checks class for sustainers in order to begin a way to easily add other sustainers checks. The plan here is to add more checks as methods and return consistent error data that can be used in watchdog, the status report, Rules, or anywhere else.
 - Rebuilds the registry in an update hook so Drupal will find the new class.
 - Runs the sustainers checks during cron, regardless of the sustainer standalone cron setting.
 - The only check right now is for stuck sustainers. These are sustainers with a lock id, a gateway response of 'success', 'failed', or null, and the attempts count != 3.
 - Sustainer records are now an entity so they have Views integration. There is now a stuck sustainer report View.
 - Rules integration for Sustainer health checks. A new Rules event plus a default rule to email the site admin on sustainer health check failures.

I decided to go with a general Sustainer health check failure event instead of one per failure. I think this makes sense because you'd probably want a single email with all the failures instead of one email per failure.

This is as opposed to the watchdog logging, which provides one message per failure type.