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

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

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

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

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