Update core.php

Added an extra parameter for conditional comments for IE.
More info: http://www.quirksmode.org/css/condcom.html

Example:
Assets::js('respond', 'media/js/respond.min.js', NULL, FALSE, 'lt IE 9');

Result:
<!--[if lt IE 9]><script type="text/javascript" src="/media/js/respond.min.js"></script><![endif]-->