com_ajax: Changes response format

Original PR: #1778
Tracker: [#31980](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31980&start=0)

As per discussion: JGEN: [Joomla Ajax Interface](https://groups.google.com/forum/#!topic/joomla-dev-general/i1syYWshGsY%5B101-125-false%5D), now new class [JResponseJson](https://github.com/joomla/joomla-cms/blob/master/libraries/cms/response/json.php) is used to build a structured response.

**Response**
```json
{
    "success": true,
    "data": "any module/ plugin data that fits JSON format",
}
```
**Exception** in com_ajax or thrown/ returned by Module/ Plugin/ Application
```json
{
    "success": false,
    "message": "Method get does not exist"
}
```

Additionally I've added options:
- `ignoreMessages` _(boolean, defaults to true)_: Set the JResponseJson to ignore dumping system messages into response->messages
- ~~`close` _(boolean, defaults to false)_: Close the application~~

In this patch Ajax Plugins are not expected to return anything - in the previous version this evaluated to an error 'plugin returned no response'.