Handle os.unlink() in our rmtree()

Did you know what on Python 3.3 on Windows os.remove() and os.unlink()
are distinct functions?

    Python 3.3.3 (v3.3.3:c3896275c0f6, Nov 18 2013, 21:18:40) [MSC v.1600 32 bit (Intel)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    Module readline not available.
    >>> import os
    >>> os.remove
    <built-in function remove>
    >>> os.unlink
    <built-in function unlink>
    >>> os.remove is os.unlink
    False