partial serialization

[seagetch]

Sometimes exporting object update via json object can be useful.
So far it always serialize all of children at once, so only if update for target object is required, always serialize all of children.
This patch introduces two functions:

1. Node.serializePartial: serialize only target object without children.
2. Node.serializeSelfImpl: called from serializeSelf and serializePartial, and serialize self based on specified recursive parameter.