update unit tests now that Call() has changed

Call() no longer outputs the incoming event.
clean up SConstruct

- add missing import that caused lib_dirs() to silently fail
- remove options that are no longer needed.
remove python 2.5 compatibility cruft
make Fork() and Chain() accept varargs

as a consequence, the remove_duplicates parameter is now keyword-only.
add varargs automatically instead of using *rest

- rename with_rest argument of @arguments.accept to add_varargs,
- eliminate the need to have an unused *rest argument in the wrapped
  function's signature.