Update Accordion.html

BUGFIX > When adding a accordion in TYPO3 backend, for each panel added, a field "title" AND a field "class Css" is purposed. However The Css field is never taken in account in the template accordion.html. 

This bugfix purpose to ad the {panel.panel.class} in the accordion template to make the css field in backend usable. (the same modification have to be done for type of fluidcontent TAB).

![bugfix-accordion](https://cloud.githubusercontent.com/assets/2372957/3711772/b2f2e95a-14e9-11e4-918a-5bc4c0dde362.png)


Best regards. 
Vincent (first bugfix of my life)