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/API/Endpoints/Setup.php

@@ -0,0 +1,24 @@
+<?php
+namespace MailPoet\API\Endpoints;
+use \MailPoet\Config\Activator;
+
+if(!defined('ABSPATH')) exit;
+
+class Setup {
+  function __construct() {
+  }
+
+  function reset() {
+    try {
+      $activator = new Activator();
+      $activator->deactivate();
+      $activator->activate();
+      $result = true;
+    } catch(\Exception $e) {
+      $result = false;
+    }
+    return array(
+      'result' => $result
+    );
+  }
+}