Allow specification of a root key on get*

This allows access to keys such as /admin/machines which are not under the
/keys path by specify the :root-key option, e.g.
(get* connection "/machines" {:root-key "admin"}).

Note that this changes the signature of key-url, so adds an overload to
preserve the previous signature of the function.
Might want to change the docstring too. :)