- 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/SendingQueue.php

@@ -59,6 +59,10 @@ class SendingQueue extends Model {
     return $subscribers;
   }
 
+  function getNewsletter() {
+    return Newsletter::findOne($this->newsletter_id);
+  }
+
   function getRenderedNewsletterBody() {
     return json_decode($this->newsletter_rendered_body, true);
   }