Install the XSL module for php5.

PR to update the main recipe to enable XSLT functions in PHP by installing the [php5-xsl package](http://packages.ubuntu.com/precise/php5-xsl). I added this as a personal project of mine was using [XSLTProcessor](http://php.net/manual/en/class.xsltprocessor.php) but usage of this extension is common enough that it might be handy to include in a default PHP environment.