Remove Entity, merge functionality into EntityManager

[dmlary]

Entity as a class is going away to support #13

• [ ] Remove VirtualComponent
  • Entity id will be a String, hashed in EntityManager
  • Entity ids will be human-readable base:room, limbo:obj/void-chest, base:obj/junk/ball-32893
• [ ] EntityManager will contain a hash with the key as Entity ID's (String), value is a hash of Component type, and Component instances.

  @entities['base:room'] = {
  ViewableComponent: #<ViewableComponent:0xfefefefe>,
  ExitsComponent: #<ExitsComponent:0xbebebebe>
  }