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/Endpoint.php

@@ -0,0 +1,25 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+abstract class Endpoint {
+
+  function successResponse(
+    $data = array(), $meta = array(), $status = Response::STATUS_OK
+  ) {
+
+    return new SuccessResponse($data, $meta, $status);
+  }
+
+  function errorResponse(
+    $errors = array(), $meta = array(), $status = Response::STATUS_NOT_FOUND
+  ) {
+
+    return new ErrorResponse($errors, $meta, $status);
+  }
+
+  function badRequest($errors = array(), $meta = array()) {
+    return new ErrorResponse($errors, $meta, Response::STATUS_BAD_REQUEST);
+  }
+}