Add NestedSets Behavior and test cases.
Add 'makeRoot' test cases.
Replace constructor with 'nestedSets' method.
Remove unused attribute.
Fix type hint.
Add 'testMakeFirstChildOf' test case.
Add docblock type hint.
Fix docblock of 'refresh' method.
Add test case of 'makeNestSiblingOf' method.
Add test case of 'makeLastChildOf' method.
Add test case of 'makePreviousSiblingOf' method.
Add test case of 'delete' method.
Add test case of 'delete' method.
Add test case of 'deleteWithChildren' method.
Add test case of 'getChildren' method.
Add test case of 'getPath' method.
Add test case of 'depth' method.