Fixing 'ls' error by adding missing OpenDir handler and mapping.

[avirtuos]

This addresses issue #150 that I raised by adding the missing OpenDir request mapping. It also adds a warn! message to stdout when an unmapped operation is attempted against the filesystem. Since the purpose of this file is to act as an example, the added visibility of what operations are being attempted (and failing) are invaluable to those using this example to learn.