Introduce get_post_ancestors(). Add current_page_ancestor class to ancestors...

Introduce get_post_ancestors().  Add current_page_ancestor class to ancestors of the current page.  Props AaronCampbell.  fixes #5662

git-svn-id: https://core.svn.wordpress.org/trunk@7074 1a063a9b-81f0-0310-95a4-ce76da25c4cd