Newsletter Segment relation

- added newsletter_segment table
- added NewsletterSegment model
- added list filter on newsletters listing
- unit tests

lib/Models/NewsletterSegment.php

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