Support deleteTree() for repositories that support getFolderTree() but not getDescendants()

The CMIS spec allows repositories to support getFolderTree() without support for getDescendants().
In the AtomPub binding deleteTree() is a DELETE request to the getDescendants() or the getFolderTree() resource. If the getDescendants() resource is not available, OpenCMIS should fall back to the getFolderTree() resource.