Added 'script_name' and 'script_args' instance attributes to Distribution.

Changed 'core.setup()' so it sets them to reasonable defaults.
Tweaked how the "usage" string is generated: 'core' now provides
  'gen_usage()', which is used instead of 'USAGE'.
Modified "build_py" and "sdist" commands to refer to
  'self.distribution.script_name' rather than 'sys.argv[0]'.