Split up how Rest::entity() and members() function are called - make separate
functions for GET, PUT, POST, and DELETE.  Among other things, this makes it
easier to describe how PUT/POST relationships work.
Debug relationships in Rest::get_resource().
Add Controller_Rest::action_put(), action_post(), and action_delete() to handle
the typical actions for a REST resource.
Revise how Controller_Rest checks for undefined actions.
Fix method capitalization in Controller_Rest::before().
Complete rewrite of Controller_Rest_Item.
- fully implemented.
- mostly, but not entirely, tested (manually).
- Model_Item::as_restful_array() logic moved to Controller_Rest_Item.
Mark REST-related unit tests as incomplete.