include upgrader timestamp

document recent changes
i18n for upgrader

lang/serendipity_lang_is.inc.php

@@ -1031,3 +1031,6 @@
 @define('INSTALLER_CLI_TOOLSTATUS', 'Executable?');
 @define('VIDEO', 'Video');
 @define('RESET_FILTERS', 'Reset filters');
+@define('UPDATE_FAILMSG', 'Check for new Serendipity version failed. This can happen because either the URL https://raw.github.com/s9y/Serendipity/master/docs/RELEASE is down, your server blocks outgoing connections or there are other connection issues.');
+@define('UPDATE_FAILACTION', 'Disable automatic update check');
+@define('UPDATE_NOTIFICATION_DESC', 'Show the update notification in the Dashboard, and for which channel?');