Add milestone labels

Usage:
```PHP
$gantt = new Gantti($data, array(
	'title'      => 'CMS'.$info["cms"],
	'cellwidth'  => 25,
	'cellheight' => 35,
	//'milestones' => array($oed, $ted)
	'milestones' => array(
		array(
			'date' => "2013-12-24",
			'label' => "Today"
		)
	)
));
```