core: Add FeedItem class (#940)

Add transformation from legacy items to FeedItems, before transforming
items to the desired format. This allows using legacy bridges alongside
bridges that return FeedItems.

As discussed in #940, instead of throwing exceptions on invalid
parameters, add messages to the debug log instead

Add support for strings to setTimestamp(). If the provided timestamp
is a string, automatically try to parse it using strtotime().

This allows bridges to simply use `$item['timestamp'] = $timestamp;`
instead of `$item['timestamp'] = strtotime($timestamp);`

Support simple_html_dom_node as input paramter for setURI

Support simple_html_dom_node as input parameter for setContent

formats/JsonFormat.php

@@ -6,7 +6,13 @@
 class JsonFormat extends FormatAbstract {
 	public function stringify(){
 		$items = $this->getItems();
-		$toReturn = json_encode($items, JSON_PRETTY_PRINT);
+		$data = array();
+
+		foreach($items as $item) {
+			$data[] = $item->toArray();
+		}
+
+		$toReturn = json_encode($data, JSON_PRETTY_PRINT);
 
 		// Remove invalid non-UTF8 characters
 		ini_set('mbstring.substitute_character', 'none');