Turn based gameplay using a scheduler.

If we don't base the game on turns/ticks, we will have problems with time coupling in the future. Notably if we want 
* multiplayer stuff.
* to allow saved games.
* to allow simulations at full CPU speed (for AI battles).
Which we want (all cases).

To do that, this introduces a scheduler that registers events in a heap and poll the heap at each turn to see which events should occur at that time.

Part of debugging the scheduler, I've created a logger that prints to a `bomb.log` file, which we can `tailf`to track what's going on.

r: @alexcoco