Update class.discussionmodel.php

GetUnread was not showing new discussions without any comment. Only discussions with at least one comment were listed

![recent](https://f.cloud.github.com/assets/3996187/871132/1a891ba6-f840-11e2-9222-c379fa969f85.jpg)
![unread](https://f.cloud.github.com/assets/3996187/871133/1ab16e9e-f840-11e2-9388-418aa0af103d.jpg)