Subcommand help, and more natural many-party introductions

Ported argument parsing to `argparse` (requires Python 2.7+) so that subcommands could be given separate help and parameter validation. Not sure whether the extra subcommand argument makes the script harder to use or not.

Also, changed the way that multiparty introductions are done, so instead of "Alice & Bob & Carol", it's "Alice, Bob & Carol".