use attached image and gallery functions from WP 3.6 if present

Discover additional eligible full-sized images in a WordPress post using new functions from WordPress 3.6, if present. 

Use [get_attached_media](http://codex.wordpress.org/Function_Reference/get_attached_media) to highlight child image attachments of the current post.

Use [get_post_galleries](http://codex.wordpress.org/Function_Reference/get_post_galleries) to extract the attachment identifiers of images in post galleries.