- Adds new models: CustomFields and SubscriberCustomField + tests

- Adjusts DB Migrator, Initializer class
- Adjusts Subscribers model tests
Closes #159

lib/Models/SubscriberCustomField.php

@@ -0,0 +1,12 @@
+<?php
+namespace MailPoet\Models;
+
+if(!defined('ABSPATH')) exit;
+
+class SubscriberCustomField extends Model {
+  public static $_table = MP_SUBSCRIBER_CUSTOM_FIELD_TABLE;
+
+  function __construct() {
+    parent::__construct();
+  }
+}