Add icons to com_banners

### How to reproduce
1) go to BE --> Components --> Banners
2) see that the icon (left the titel is missing)
3) swith to the clients view
4) note that there the icon is also missing
5) apply patch
6) note that the icons will show

### Bevor the patch:
![missing_icon_banners](https://cloud.githubusercontent.com/assets/2596554/3044330/bc343292-e110-11e3-8eb9-4c4a1f0ae47c.JPG)
![missing_icon_clients](https://cloud.githubusercontent.com/assets/2596554/3044331/bc347ec8-e110-11e3-9e79-5035a8b86e6d.JPG)

### After the patch:
![fixed_icon_banners](https://cloud.githubusercontent.com/assets/2596554/3044329/bc30a85c-e110-11e3-96b7-18de786b2182.JPG)
![fixed_icon_clients](https://cloud.githubusercontent.com/assets/2596554/3044332/bc34d300-e110-11e3-8460-a57815f291e5.JPG)

### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33767&start=0