Responsibility to start and end game

I think it is more reasonable for the game engine to start and end the game, rather than having an agent send requests for starting and ending the game. Once 4 or 5 players are added, we could start the game; once a certain number of rounds is reached (or if someone runs out of money), we could end the game.

Also, it would be nice if the number of seconds of a round is customizable (i.e. set as a constant in the constants.py file). This will facilitate debugging.