- Updates cron's sending queue worker to use model objects

- Adds new method to render newsletter to the newsletter model
- Adds new transient object to newsletter model that will hold temporary
  values (i.e., rendered body) when working with the model

lib/Models/Newsletter.php

@@ -1,12 +1,13 @@
 <?php
 namespace MailPoet\Models;
+use MailPoet\Newsletter\Renderer\Renderer;
 use MailPoet\Util\Helpers;
 
 if(!defined('ABSPATH')) exit;
 
 class Newsletter extends Model {
   public static $_table = MP_NEWSLETTERS_TABLE;
-
+  public $_transient;
   const TYPE_STANDARD = 'standard';
   const TYPE_WELCOME = 'welcome';
   const TYPE_NOTIFICATION = 'notification';
@@ -22,7 +23,7 @@ class Newsletter extends Model {
 
   function __construct() {
     parent::__construct();
-
+    $this->_transient = new \stdClass();
     $this->addValidations('type', array(
       'required' => __('Please specify a type')
     ));
@@ -246,6 +247,11 @@ class Newsletter extends Model {
     return $this;
   }
 
+  function render() {
+    $renderer = new Renderer($this);
+    return $renderer->render();
+  }
+
   function getStatistics() {
     $statistics_query = SendingQueue::tableAlias('queues')
       ->selectExpr(