adjusting handler of 'active' shortcode attribute to handle FALSE value

As per http://jqueryui.com/accordion/#collapsible and http://api.jqueryui.com/accordion/#option-active, if 'active' is set to FALSE and 'collapsible' is set to TRUE, the accordion should be initialized with every panel closed. This commit adds that functionality. Setting 'active' to anything other than false or an integer will revert to the default value and initialize with the first panel open.