Support JSONP callback functionality in JSON query results

JSONP (http://en.wikipedia.org/wiki/JSONP) is an extension of JSON to support callback functions, allowing Javascript/JQuery clients to do cross-domain Ajax calls. See http://stackoverflow.com/questions/10784271/load-srj-files-with-jquerys-ajax-or-getjson. To support this, we should read the optional 'callback' parameter whenever a JSON-formatted result is request, and pad the JSON-serialized query result with the callback function.