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/Settings.php

@@ -0,0 +1,27 @@
+<?php
+namespace MailPoet\API\Endpoints;
+use \MailPoet\API\Endpoint as APIEndpoint;
+use \MailPoet\Models\Setting;
+
+if(!defined('ABSPATH')) exit;
+
+class Settings extends APIEndpoint {
+  function __construct() {
+  }
+
+  function get() {
+    $settings = Setting::getAll();
+    return $this->successResponse($settings);
+  }
+
+  function set($settings = array()) {
+    if(empty($settings)) {
+      return $this->badRequest();
+    } else {
+      foreach($settings as $name => $value) {
+        Setting::setValue($name, $value);
+      }
+      return $this->successResponse(Setting::getAll());
+    }
+  }
+}