lib/bup/vfs: Add docstrings

lib/bup/vfs: Add docstrings

Since the vfs module uses the function git._treeparse, it should not be
named as if it was a private function. Rename git._treeparse to
git.treeparse and document it (add a docstring to it).

Also, transform _ChunkReader, _FileReader and Node into new-style
classes.

Finally, remove trailing spaces from lib/bup/vfs.py .