Added API/Endpoint abstract class

- (re)Added Endpoints folder to both API and Router
- fixed syntax in namespaces
- xhr.responseJSON is returned to the fail()
- fixed Router endpoints (view in browser, cron,...)

lib/Cron/CronHelper.php

@@ -2,7 +2,7 @@
 namespace MailPoet\Cron;
 
 use MailPoet\Router\Front as FrontRouter;
-use MailPoet\Router\Queue as QueueEndpoint;
+use MailPoet\Router\Endpoints\Queue as QueueEndpoint;
 use MailPoet\Models\Setting;
 use MailPoet\Util\Security;