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 follow Node.JS's with a little change in
its result to make it fit in well with Java File API.