Rename root directory to ""
Make walk recursive

Add support for bottom-up walk
Update parents of attached files and dirs
Update attach to work with new walk

The base path in the new walk is now always
absolute
Better __str__ for dirs and files
Swap argument order
Change parent when unattaching
Add line breaks between methods
Fix typo
Create vdir __iter__
Add visualize
Properly indent file names
Rename visualize to ls and make recursion optional
Move ls
Made ls accept an optional stream

All output is sent to stream. stream defaults to sys.stdout
Added run-through tests for walk and ls