Added option --update (skip files that are newer on the receiver)

Added option --update

You can use it like this:

```php
$config = array(
    'update' => true, 
);

$rsync = new Rsync($config);
```

or


```php
$rsync = new Rsync;
$rsync->setSkipNewerFiles(true);
```

Thank you for the lib btw.