com_ajax: Changes in how response is built

As per discussion: JGEN: [Joomla Ajax Interface](https://groups.google.com/forum/#!topic/joomla-dev-general/i1syYWshGsY%5B101-125-false%5D), [JResponseJson](https://github.com/joomla/joomla-cms/blob/master/libraries/cms/response/json.php) is now used to build structured response.

Additionally I've added options:
- `ignoreMessages` _(boolean, defaults to false)_: Set the JResponseJson to ignore dumping system messages into response->messages
- `close` _(boolean, defaults to false)_: Close the application

Ajax Plugins are not expected to return anything. In the previous version this set an error 'plugin returned no response'.