Make ObjectId support JSON.stringify.
Add array type field.

See https://github.com/SollmoStudio/beyond/wiki/Beyond-DB-API-Schema
for further information.
Support optional field for DB schema API.

Currently, if there is an undefined field when inserting a document, the
document cannot be saved.

This patch makes ScriptableDocument support the optional field for DB schema.
Support default field for DB schema API.

A field with default value can be null when inserting a document. The default
value is not saved in DB, it will used on read.