A node must keep its position relative to its siblings when being renamed

Fix #255

Test added in https://github.com/phpcr/phpcr-api-tests/pull/144
this is maybe not the most elegant way of solving this, but the impact is low and i see rename as a shortcut for this operation anyways. and lacking native support in the transport layer, this would just end up the same way if we would add an internal method that unifies move and rename. (Session::move does a couple of important checks to avoid name collisions).