Add current case to user form field

This adds a current case to the user form field.

I intially put this as a tracker here ( http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30051 ) - but it got incorrectly closed - as this is not a platform form field. Then I realised I put the pull in my master branch of my fork. So have created a new pull/tracker item to fix everything up :)

New tracker item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30455