involve a filesystem path.  To that end:

- Changed IOError to EnvironmentError and added a hack which checks
  for arg of len 3.  When constructed with a 3-tuple, the third item
  is the filename and this is squirreled away in the `filename'
  attribute.   However, for in-place unpacking backwards
  compatibility, self.args still only gets the first two items.  Added
  a __str__() which prints the filename if it is given.

- IOError now inherits from EnvironmentError

- New class OSError which also inherits from EnvironmentError and is
  used by the posix module.