Add "added" timestamp to song database

- added is set to current time, if a new song is added to the database.
- GetAdded falls back to mtime.

Code for proxy plugin is missing, this needs a patch for libmpdclient.

closes #378

src/command/QueueCommands.cxx

@@ -276,6 +276,9 @@ ParseSortTag(const char *s)
 	if (StringIsEqualIgnoreCase(s, "Last-Modified"))
 		return TagType(SORT_TAG_LAST_MODIFIED);
 
+	if (StringIsEqualIgnoreCase(s, "Added"))
+		return TagType(SORT_TAG_ADDED);
+
 	if (StringIsEqualIgnoreCase(s, "prio"))
 		return TagType(SORT_TAG_PRIO);