- Added Sudzy composer dependency for model validation

- All models now extend a generic Model class that extends Sudzy
- Wrote tests for Subscriber and Setting model validation
- Rewrote model tests to make sure they look the same
- Updated both tests with cleanup logic in the beginning
- Added test:unit-single method to Robo for selective unit testing

Closes #54

lib/Models/Model.php

@@ -0,0 +1,13 @@
+<?php
+namespace MailPoet\Models;
+
+if (!defined('ABSPATH')) {
+  exit;
+}
+
+class Model extends \Sudzy\ValidModel {
+  function __construct() {
+    $customValidator = new CustomValidator();
+    parent::__construct($customValidator->init());
+  }
+}