Update shellista.py

various fixes and minor additions, including:
* replaced `.rstrip('/')` with `os.path.normpath()` to fix handling of paths like `/`
* simple OSError handler in `do_ls` to avoid crashing the shell when ls'ing a forbidden file
* documented/added help text for a couple commands and aliases
* minor code style things here and there
Merge pull request #1 from dgelessus/patch-1

Update shellista.py