Added storing current and next page numbers parsed from HAL links

They are now available in Collection objects so that you can iterate through endpoint's pages easily e.g. like this:
```
<?php
$config = $this->getServiceLocator()->get('Config');
$client = \SvpApi\Client::factory($config['sch']['vgtv']);

$limit = 10;
$page = false;

do {
    $assets = $client->fetchAssets($limit, $page, 'categoryId::1');
    $page = $assets->getNextPage();

    foreach ($assets as $asset) {
         //...
    }    
} while ($page);
?>
```