Add an ID value to each player

This is a first baby step toward implementing the more complete API
for the game which we defined today, and which can be found here:

	https://git.cworth.org/git/empires-api

In this commit we also introduce a 'Game' class to encapsulate all
data necessary for the game, (so the 'players' array we had before,
but also the 'next_player_id' which we need now).