Add JResponseJson as a CMS library class

This adds the generic Json response class that is located at the helpers of the languages component as a new CMS library class with the name JResponseJson. so it can easily also be used by other extensions also because it is auto-loadable now.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31289