Tweaks

Small bug in the AI where it would ignore skull matches instead of avoiding them. Some graphics tweaks. Small fixes.

Still reviewing high level play - it's possible to end up with no matches and 6 moves around the 2 x dead coin mark. Considering offering a bomb instead of a Hint, not the direction I want to go in but the unsolvable-board state still needs something conclusive. Simply asking the player to do the death march is not that cool. I'd prefer to avoid a shuffle as well - it becomes a tactic in games like Candy Crush when you've made a mess of things. I want the player to be more involved with tying up lose ends.