modify test_folder_hierarchy.py (topdown/bottomup)
merge getlist and execQuery that uses printCallback
add walk_iter() in googleDriveAccess.py
add walk_visit() in googleDriveAccess.py

walk_visit() uses visitCallback() like os.path.walk()
walk_iter() yields iterable information like os.walk()
Merge pull request #6 from HatsuneMiku/master

(topdown / bottomup) and (walk / walk_visit / walk_iter)