they make sure that 'set_final_options()' has been called, but isn't
  called redundantly.
Changed Distribution to call 'ensure_ready()' where it used to call
  'set_final_options()', and in a few extra places as well.
Lots of comment/docstring revisions and additions in both classes.
New one-liner utility methods in Command: 'find_peer()', 'spawn()'.