Added "overlay" functionality for player/NPCs.

New tileset IDs:
- overlay_worn_{ITEM ID}, overlay drawn over the character when the item is worn
- overlay_wielded_{ITEM ID}, overlay drawn over the character when the item is wielded

Tested this by modifying existing tileset.
```json
        {
          "rotates": false,
          "id": "overlay_wielded_crowbar",
          "fg": 2470,
          "bg": 2
        },
        {
          "rotates": false,
          "id": "overlay_worn_sunglasses",
          "fg": 2471,
          "bg": 2
        },
        {
          "rotates": false,
          "id": "overlay_wielded_sunglasses",
          "fg": 2472,
          "bg": 2
        }
```
Wielded crowbar, worn sunglasses:
![wielding_crowbar](https://cloud.githubusercontent.com/assets/902211/4264353/455e1e80-3c23-11e4-97ab-dae176a6676e.png)

Wielded crowbar:
![wielding_crowbar](https://cloud.githubusercontent.com/assets/902211/4264358/4ef81c98-3c23-11e4-9479-8ebdaf772a8b.png)