Improve AI

[tomchen]

To improve the AI, actually I think the AI is OK now, at least it's better than the original game's that's sometimes suicidal. However, there are indeed something to do, including:

• It should be able to see how to win within 2-4 steps and do it (more strategical)
• If it's about to win. It should chase the win. Especially resources, if res is about to reach the winning cond, it shouldn't spend it
• If it's about to lose (player's tower or res is too high), it should priorize attacking cards (even if the card damages the AI itself, or benefits the player in terms of non-tower elements)
• If it's about to lose (its tower is too low), it should priorize tower-adding cards (even if the card benefits the player, or damages the AI itself in terms of non-tower elements)
• It should loop through all cards and see if a winning card exists, then use a tie card (it shouldn't see a tie card and use it immediately without seeing all cards)

See also #45 (Add AI level settings)