Accessibility fixes for advanced search form.

Hi all, 

Just some small changes for the sake of accessibility:
*Changed the #advanced-form fieldset to a div since its role was more structural than semantic.
*Use legends inside of fieldsets instead of paragraph tags. See: http://www.w3.org/TR/2008/WD-WCAG20-TECHS-20080430/H82.html
*Added labels associated with checkboxes.