Better art, and better game balance

We've got a slightly prettier Undead, with a new Icon for the Demi-Lich and pixel art for all unique units. But better yet, game balance! The Demi-Lich is more expensive and grows in cost as you conquer more cities, units cost a little more in maintenance, all cities have a little less food and gold (but a little more happiness), air units don't get a bonus against cities, and the only units that heal when they kill are melee units (with the exception of the Bone Dragon, because bone dragons are awesome). Enjoy!