Notifiers callbacks

```atoum\report\fields\runner\result\notifier\terminal``` now supports a callback command which is executed when the user clicks on a notification.

Here is an example configuration file:

```php
<?php

use \mageekguy\atoum;

$logFile = '/tmp' . DIRECTORY_SEPARATOR . sprintf('atoum-%d.log', time());

$notifier = new atoum\report\fields\runner\result\notifier\terminal();
$notifier
    ->setCallbackCommand('qlmanage -p ' . $logFile . '; rm -f ' . $logFile . ' *> /dev/null')
;

$growl = new atoum\report\fields\runner\result\notifier\image\growl();
$growl
    ->setCallbackUrl('file://' . $logFile)
    ->setSuccessImage(__DIR__ . '/resources/images/logo/success.png')
    ->setFailureImage(__DIR__ . '/resources/images/logo/failure.png')
;

$cliReport = new \mageekguy\atoum\reports\realtime\cli();
$cliReport->addWriter(new atoum\writers\file($logFile));
$runner->addReport($cliReport);

$script->addDefaultReport()
	->addField($growl)
    ->addField($notifier)
;

```