v0.4.1

Added

• provide FrameDesc with a Default impl to make generating empty
  descs for rx simpler

Fixed

• negate error codes when calling io::Error::from_raw_os_error
• some libc calls just return -1 on error, not an informative
  error code so in these cases call io::Error::last_os_error()
  instead of io::Error::from_raw_os_error(err), where err is
  always equal to -1...