Log Message:
Update to 4.0.1:

    The build will now fall back to pure-python mode if the C
    extension fails to build for any reason (previously it would
    fall back for some errors but not others).
    IOLoop.call_at and IOLoop.call_later now always return a timeout
    handle for use with IOLoop.remove_timeout.
    If any callback of a PeriodicCallback or IOStream returns a
    Future, any error raised in that future will now be logged
    (similar to the behavior of IOLoop.add_callback).
    Fixed an exception in client-side websocket connections when
    the connection is closed.
    simple_httpclient once again correctly handles 204 status codes with no \ 
content-length header.
    Fixed a regression in simple_httpclient that would result in
    timeouts for certain kinds of errors.