Replace current email functions with PHPMailer

PHPMailer is an popular open source full-featured email creation and transfer class for PHP. It's used by many open-source projects: Drupal, SugarCRM, Yii, Joomla! and many more. So I think it would be better OpenCart can keep pace with the others as well.

PHPMailer page: https://github.com/PHPMailer/PHPMailer