implement player loses on move timer expires

Fixes #11.

game/game.go

@@ -240,6 +240,11 @@ func (g *Match) Concede(id player.ID) error {
 	return ErrRoundEnded
 }
 
+// Expire makes the current player concede.
+func (g *Match) Expire() {
+	g.Concede(g.CurrentPlayer().id)
+}
+
 // DTO returns the current match state as viewed by the given player and with
 // the given deadline on the current player's next move.
 func (g *Match) DTO(id player.ID, deadline time.Time) serve.Game {