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/Subscription/Pages.php

@@ -1,7 +1,6 @@
 <?php
 namespace MailPoet\Subscription;
 
-use \MailPoet\Router\Subscribers;
 use \MailPoet\Models\Subscriber;
 use \MailPoet\Models\SubscriberSegment;
 use \MailPoet\Models\CustomField;