Wrong 'yes/no' class for grouped btn


![before_plg_btn](https://f.cloud.github.com/assets/1972717/1416091/633dcfda-3f36-11e3-9f70-7354aa037527.png)

The group btn class for yes/no ‘Strong Passwords' buttons should be:
"btn-group btn-group-yesno"

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32445&start=0