Nutree

- BREAKING: changed `tree.load()` / `tree.save()` signature, and storage format.
- Support load/save for TypedTree

- BREAKING: Rename `node.move()` -> `node.move_to()`
- New `tree.copy_to()` and `node.copy_to()`
- New `tree.children` property
- Configurable default connector style

Initial releases.