- 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/CustomValidator.php

@@ -0,0 +1,19 @@
+<?php namespace MailPoet\Models;
+
+class CustomValidator {
+
+  function __construct() {
+    $this->validator = new \Sudzy\Engine();
+  }
+
+  function init() {
+    $this->validator->addValidator(
+        'isString',
+        function ($val) {
+          return is_string($val);
+        }
+    );
+
+    return $this->validator;
+  }
+}