A bunch of refactoring

![Arnold-Schwarzenegger](https://f.cloud.github.com/assets/19938/185456/94afb3cc-7d07-11e2-98c5-bba8ce4d0534.jpeg)

* Introduced **_get_mail_object**
* Renamed old **_get_mail_object** to **_get_mail_class**
* Added optional lang parameter to render parts and override the default **get_language**.