unittest: Add support for SkipTest, skip() and skipUnless().
os: Add access().
os.path: Add dummy normpath() & join(), working split().
os.path: Add exists().
os: Add makedirs().
os: Add os.error as an alias for OSError.
os: Fix listdir() error raising.
os.path: Define lexists as an alias to exists for now.
os: Import _os for C-level functions.
os: Reimplement listdir() in terms of iterator.
os: Implement walk().
os: Add unlink() & rmdir().
os: Add getcwd().
os: Rename direcorty functions test.
os.path: Implement isdir().
os.path: Add poorman's abspath().
test.support: Addmore functions.

can_symlink() so far returns False for simplicity.
os: Add sep, curdir, pardir constants.
shutil: Decided to reimplement, start with shutil().
stat: Add from CPython 3.3.3.