Fixes broken Datatable Checkbox example

See current broken example here: http://yuilibrary.com/yui/docs/datatable/datatable-chkboxselect.html

Made the example start with no checked boxes so that the code can properly track the current state of the datatable checkboxes, as designed by the original author . I also added the silent to stop it from having weird effects when trying to unclick a checkbox (see above link).