Add ajax helper

When using ajax requests, it's often helpful to have a helper that manages the results, sending data to client in json format.
This helper offers two methods, `sendResult` and `sendError`, which sends the result or the error message to the client and ends the execution.

Usage on server side:
```php
Loader::helper('ajax')->sendResult(...);
Loader::helper('ajax')->sendError('Error message.');
```
On the server site you can have a javascript like this:
```javascript
$.ajax({
   url: ...,
   dataType: "json",
   success: function(data) {
   },
   error: function(xhr) {
      alert(xhr.responseText);
   }
});
```