[WIP] Prevent creating same name siblings

Added "clone" test cases for existing, non-corresponding nodes in the destination workspace.

Attempting to clone over an existing node with a different UUID should result in `PHPCR\ItemExistsException` being thrown, with or without removeExisting=true.

As discussed here:
https://groups.google.com/forum/?fromgroups=#!topic/symfony-cmf-devs/Ro12vnxInG8