Commit of partial work (this commit will be amended) to share with James

    - during validation stage:
        Event 1: resource_valid
            - run on each Module in URL path
            - Does this event need a better name, like validate_resource ?

            - binders:
                - cb_users_*
                    - load user_id into context
                - cb_accounts
                    - load account_db/accout_id into context

        Event 2: validate
            - only run on last Module, like resource_exists

    If only resource_exists() method took Context as parameter, could fold over