ArrayNode: make _children final

Simplifies quite a lot of code in the process.
ObjectNode: make _children final

Simplifies quite a lot of code in the process.
ArrayNode: remove unneeded constructor

The List container now being final, we don't need this constructor anymore.

Just create a base ArrayNode and .add() elements to _children instead.
ObjectNode: remove unneeded constructor

The Map container now being final, we don't need this constructor anymore.

Just create a base ObjectNode and .put() entries into _children instead.
Merge pull request #153 from fge/master

Looks good, will merge -- ObjectNode and ArrayNode: make _children final