Arrow/Enemy collision fixed; Items dropped upon enemy death

- when arrows hit enemies they become stuck to enemy's x & y; they disappear after 5 seconds or so
- arrows can no longer be collided with after they are stuck in something
- Commodity class added to handle collectible items (this is probably just a temporary)
- 50% of enemies drop items upon death (keys, arrows, coins). At the moment only the arrows are possible to collect