Add ability to retrieve URL report URL for download

[tegansnyder]

This adds the ability to retrieve the URL to download the URL report. Example usage:

$diffbot = new Diffbot(MY_TOKEN_HERE);
$crawl = $diffbot->crawl('SomeCrawlNameHere');
$url_report = $crawl->getUrlReportUrl();

// show the url to the url report
echo $url_report;

[Swader]

Brilliant, thanks