Spectator feature

This adds support for spectators to the game.

Spectators can view and chat in a game but they can't play cards or be chosen as czar.  The game host may separately configure the number of allowed spectators (including down to zero if desired).

I'm not entirely happy with the scoreboard mechanic -- I gave spectators a scoreboard panel just so that players can see who is spectating, but obviously it's a bit of a waste of space.  Not sure if there's somewhere better to show them.

Similarly (future expansion maybe) I was thinking that it might be a good idea to have separate passwords for playing and spectating.  That way someone could set up a game that's private for their friends to play in, but public for anyone to watch.

But as it stands it seems to work fine.

Note that this depends on the changes in pull request #43 (otherwise things get ugly if the last player leaves a game before the spectators), so you should merge that in first.