private String getJsonCallback() {
        Result result = getResult();
        Query query = result.getQuery();
        if (query != null) {
            return query.properties().getString(JSON_CALLBACK, null);
        }
        return null;
    }
Adds JSONP (Json with padding) support.

Basically, if the JSON renderer receives a query parameter "jsoncallback=...",
the JSON response will be wrapped in a function call with the name specified
by the client. This side-steps the same-origin policy, thus supports calling
Vespa from javascript loaded from a different domain then the Vespa instance.
container-search/src/main/java/com/yahoo/search/rendering/JsonRenderer.java