[WIP] Prevent creation of same name siblings when cloning nodes

The Jackalope ObjectManager now handles most cases when attempting to clone over an existing node.

Note, the tricky case I mentioned in [this topic](https://groups.google.com/d/msg/symfony-cmf-devs/Ro12vnxInG8/C9e8Fz_MoHIJ) proved to be unnecessary, because Jackrabbit returns a HTTP 409 in that case.

Still todo (here and/or in the tests): cloning from a non-referenceable node to a referenceable node should throw an exception.