Extend newObject and newArray methods of Rhino Context.

Currently, arguments must be wrapped in an array when using newObject
and newArray method. It makes code verbose.
Refactoring code to use ContextOps.

The previous patch impelents ContextOps to extend newObject and newArray
methods. This patch makes code use ContextOps.
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.
Merge branch 'master' of github.com:SollmoStudio/beyond