A bunch of cool changes

* Switched the nonce check from `check_admin_referer` to `check_ajax_referer`
* Refactored `plupload_image` field so the code is a lot cleaner and leaner.  A lot of cruft removed
* The `posts` field now has an option called `parent`, which can be set as `true` or `false` (default).  If set to true, instead of saving the result as meta data, the post you choose is set as the `post_parent` of the current post.  In other words, you can create parent-child relationships, even across post types