Do not convert to NativeObject if it is a primitive type.

Currently, each field type of Document is always 'object' instead of the
expected type (e.g., 'string', 'number' and 'boolean') because
ScriptableDocument converts it to NativeObject.

This patch makes ScriptableDocument return the primitve type with no
conversion.  Rhino engine will automatically convert Number, Boolean and String
because BeyondContextFactory set javaPrimitiveWrap to false.
Don't share emptyUpdatedValueTable across multiple ScriptableDocument.

Currently, ScriptableDocument's updateValues field accidentally shares a
single UpdatedValueTable because emptyUpdatedValueTable is a singleton defined
in ScriptableDocument's companion object. Move it to ScriptableDocument class
so that each ScriptableDocument instance has its own UpdatedValueTable.

Also move UpdatedValueTable type to ScriptableDocument class because it is used
only within ScriptableDocument class.
Expose objectID as objectId in JavaScript.

Follow the mongo shell's naming convention.
Make collection.save method return a Future of the saved document on success.

Currently,  save method returns a Future of LastErrorMessage. But the updated
document is needed in most cases.

This patch makes save method return a Future of the saved document on success.
Implement ScriptableFileSystem.

It's an interface to provide JavaScript API for handling file system.
Now only basic functions are implemented to read and write files.
The API design is found in the Wiki page below.

https://github.com/SollmoStudio/beyond/wiki/Beyond-File-System-API
Add ScriptableFile.

ScriptableFile is a wrapper of Java File class for Rhino. Currently only
`name`, `path`, `isFile` and `isDirectory` getters are implemented.
Implement `readdir` method for file system API.

`readdir` API reads all files from a provided directory path, and return
a future object of an array of ScriptableFile objects.

The method name and API design follows Node.JS's with a little change in
its result to make it fit in well with Java File API.