Allow tracking of entities by reference (in addition to name)

Adds following of entities by reference. Useful for unnamed entities. For example:
```javascript
var parameters = {text: 'Hello!', tracks: this};
ig.game.spawnEntity(EntityChatbubble, 0, 0, parameters);
```

Retains original API where name can be used, and that the name can be
changed at any point (inefficient but retains API).