Added Difficulty

Added 3 difficulties to pick from when starting a new game. The starting stats for each are as follows:

Easy :  100 Population, 40 Houses, 350 Food, 280 Water, and 150 Gold

Medium: 50 Population, 20 Houses, 175 Food, 200 Water, and 75 Gold

Hard:  5 Population, 2 Houses, 30 Food, 20 Water, and 50 Gold