Implemented enemy item drop, for now Items are implemented as regular entities with an ItemController. Entities, when defined in json, can specify an item attachment (the name of the item entity). (This necessitates two passes through the entity definition jsons). When an enemy is killed, in onDamage, the itemDrop is spawned if that entity has a itemDrop defined. Otherwise, the enemy can die without dropping.

May want to also specify a drop chance or a list of possible drops (in which case, we might want a specialized item object to separate logic). We way want specialized item objects either way to better encapsulate their logic). Thoughts?