Add ScriptableFile.

ScriptableFile is a wrapper of Java File class for Rhino. Currently only
`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.