New add_candidate option when configuring SmartID, to allow the user to decide...

New add_candidate option when configuring SmartID, to allow the user to decide whether to prepend or not the candidate attribute name to the resulting value. Issue #605.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3318 44740490-163a-0410-bde0-09ae8108e29a