More on issue #4: Work toward Python 2 & 3 compatibility.

This doesn't get all the way to full bidirectional compatibility, but
it's progress toward that goal.  Run 'make check' to see what's next.

* onetime: Add 2 vs 3 conditional block at top.
  (btos): New utility function, defined in above block.
  (Pad.convert, Pad._make_fuzz): Handle string vs int manually.
  (main): Use binary_stdout.  Encode headers and footer before writing
    them.  Use new btos() helper where necessary.