Unify link tag for RSS & Atom into $url property (#20)

In rss we have tag <link>https://example.com/articles/some-amazing-article</link>,
but in Atom we have <link href="https://example.com/articles/some-amazing-article"/>.
So, lets extract it from href attr & put into `url` property.

readme.md

@@ -30,11 +30,11 @@ the information from the channel is easy:
 ```php
 echo 'Title: ', $rss->title;
 echo 'Description: ', $rss->description;
-echo 'Link: ', $rss->link;
+echo 'Link: ', $rss->url;
 
 foreach ($rss->item as $item) {
 	echo 'Title: ', $item->title;
-	echo 'Link: ', $item->link;
+	echo 'Link: ', $item->url;
 	echo 'Timestamp: ', $item->timestamp;
 	echo 'Description ', $item->description;
 	echo 'HTML encoded content: ', $item->{'content:encoded'};

src/Feed.php

@@ -81,7 +81,8 @@ private static function fromRss(SimpleXMLElement $xml)
 			// converts namespaces to dotted tags
 			self::adjustNamespaces($item);
 
-			// generate 'timestamp' tag
+			// generate 'url' & 'timestamp' tags
+			$item->url = (string) $item->link;
 			if (isset($item->{'dc:date'})) {
 				$item->timestamp = strtotime($item->{'dc:date'});
 			} elseif (isset($item->pubDate)) {
@@ -102,8 +103,9 @@ private static function fromAtom(SimpleXMLElement $xml)
 			throw new FeedException('Invalid feed.');
 		}
 
-		// generate 'timestamp' tag
+		// generate 'url' & 'timestamp' tags
 		foreach ($xml->entry as $entry) {
+			$entry->url = (string) $entry->link['href'];
 			$entry->timestamp = strtotime($entry->updated);
 		}
 		$feed = new self;