Update VFS-local to work on Windows

This PR starts to introduce some changes to let VFS work on Windows. 

There are four tests not passing:

`vfs.rmfile`
`vf.rmdir`
`vfs.symlink`
`vfs.execFile`

Most of these are permission settings--I am getting `EPERM` errors for some reason. This is compatible with errors I saw in https://github.com/c9/vfs-local/pull/7.

This should also fix #1. It's not that it doesn't work on Windows, it's that the testing infrastructure makes too many incorrect assumptions about dir paths. Everything else is golden since it relies on `fs`.