adding wpsc_purchlogitem_metabox_end hook to item-details.php

We're trying to stop non-link items from getting hooked to wpsc_purchlogitem_links_start by providing a specific spot to hook non-link meta.

See Github issue
https://github.com/wp-e-commerce/WP-e-Commerce/issues/122

Basically just making a proper spot for developers to hook in to when
they want to add non-link metaboxes.