Freebird: the new go-recurly plugin

mostly pulled out of `go-subscriptions` and `go-subscriptions-freebies`.

see https://github.com/GigaOM/legacy-pro/issues/4125
These may be in here for some kind of record, but the whole second section can just be `default: break;` - then you are whitelisting the first portion and breaking on everything else - which is what is going on here, but there is no obvious default to handle things that aren't in the two case lists.

```
switch ( $notification->type )
		{
			case 'new_subscription_notification':
			case 'expired_subscription_notification':
			case 'successful_payment_notification':
			case 'renewed_subscription_notification':
			case 'updated_subscription_notification':
				$this->recurly_sync( $user );
				break;
			// we don't really care about synchronizing recurly data for the other types
			default:
				break;
```