[TASK] Render "class" into HTML

The field "Additional CSS class names" wasn't rendered into the HTML.
May I propose to add the class to the tabs as well as to the content div?

• tab-pane-{tab.tab.class}
• tab-{tab.tab.class
Merge pull request #96 from abteilung/patch-2

[TASK] Render "class" into HTML