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,...)

tests/unit/API/NewslettersTest.php

@@ -1,5 +1,5 @@
 <?php
-use \MailPoet\API\Newsletters;
+use \MailPoet\API\Endpoints\Newsletters;
 use \MailPoet\Models\Newsletter;
 use \MailPoet\Models\NewsletterSegment;
 use \MailPoet\Models\NewsletterTemplate;