Use the equivalent of JResponseJson

This PR introduces a backported JResponseJson class to produce the repsonse. Note this will require a major version bump as the json structure returned is different (and not b/c compatible) with that returned now.

This is compatible back to PHP 5.0 (so some Joomla 1.5 installs might be at risk) as Exception class didn't exist before then. Ditto with the constructor method. 

Note I need to test this before it gets merged. This is just a first pass at getting it implemented.