Replaced Object.build for new
Added Alive mixin and inputSources.Keyboard
Added a second player, just because.
Updated Dog to use Alive

* Removed not used code from Dog
* Alive uses a Null sourceInput as default
Added Function.prototype.includeMixin utility

* Using the new Function method to include mixins in Dog and Player.
Introduced Entity abstraction
Added Zombie and basic AI for it.

* Added Zombie
* Added Enemy inputSource.
* World now passes a `state` to the entities. Entities pass this state
on to their respective inputSources.