Calling $parent->append($node) and $parent->prepend($node) now automatically
saves $node. Those functions returns whether the node was saved.
ancestorsOf now return ancestors only, not including target node.
Default order is not applied automatically, so if you need nodes to be in tree-order
you should call defaultOrder on the query.
Since root node is not required now, NestedSet::createRoot method has been removed.
NestedSet::columns now doesn't create a foreign key for a parent_id column.