Fixes for no-level running

• Fix updateNode(): don't increment level if no level column.
• Allow level to be null if it's virtual.
• Add more tests.

Fix getAncestors() and more tests

• Fix getAncestors() for depth greater than 1.
• Add more tests (isEqualTo, isDescendantOf, isDescendantOfOrEqualTo, isAncestorOf).

Optional level field, add parentId field support

See #12 and #6