Enemy health and dying

We'll need to be able to track enemy HP. I haven't listed this as depending on the #1 or #3 because all this stuff should be testable without any enemies or players.

- [x] Enemies should be able to be damaged by the player's attacks. (Which are not confined to a grid; hit detection will be using Unity's normal collider system.)
- [x] When an enemy is hit, there should be some kind of visual effect to make it clear that specifically they got hit.
- [x] When an enemy dies, some sort of effect (beyond just vanishing) should exist.