rename need_navigation into paginable
paginable has been recently introduced by changeset d4eeaf0393e2.
We already had the need_navigation attribute to control the same
thing (pagination). As paginable sounds like a better name, deprecates
need_navigation (bw compat ok).
Also:
* fix main pagination functions responsability
* deprecate limit_rset_using_nav in favor of paginate/do_paginate
* navigation module should be renamed to pagination at some point