- Adds new models: CustomFields and SubscriberCustomField + tests

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

lib/Models/CustomField.php

@@ -0,0 +1,25 @@
+<?php
+namespace MailPoet\Models;
+
+if(!defined('ABSPATH')) exit;
+
+class CustomField extends Model {
+  public static $_table = MP_CUSTOM_FIELDS_TABLE;
+
+  function __construct() {
+    parent::__construct();
+
+    $this->addValidations('name', array(
+      'required' => __('You need to specify a name.')
+    ));
+  }
+
+  function subscribers() {
+    return $this->has_many_through(
+      __NAMESPACE__ . '\Subscriber',
+      __NAMESPACE__ . '\SubscriberCustomField',
+      'custom_field_id',
+      'subscriber_id'
+    );
+  }
+}