Atom support

[vbauer]

Could you please add Atom support? I've found that 'pubDate' parameter is empty for atom feeds.

UPD: Some RSS feeds use "pubDate" instead of "published" or "updated", ex: http://www.developer.com/developer/iworld-ws-android.rss

Fix is pretty simple:

    private function getPublishedDate($item) {
        $pubDate = $item->published;
        if (!$pubDate) {
            $pubDate = $item->updated;
        }
        if (!$pubDate) {
            $pubDate = $item->pubDate;
        }
        return (string)$pubDate;
    }