toArray added option excludeRoot

Not in all cases a Nested Set was one root item.
This patch adds a `excludeRoot` option to the `toArray` function to allow a this.

Other minor tweaks are:
- make the root item use a Number `left` not a string
- make the root item use `parent_id: null` since this is the default behavior when using the `excludeRoot` option (and in my opinion is more logical then `'none'`)