[#70] Improve Full State update efficiency

* Refactoring: Introduce Beer class to handle spawning of beers in gameworld.
* Creating a new Beer set defaults in the constructor
* Refactor to make serialization a separate method.
* Add underscore as a shim for client side code.
* Refactor the rendering order so that beers are considered Powerups.
* Message serialization for beers is now greatly reduced.
* Entities can now have default images that can managed in code, rather than passed on values via JSON.
* Add a 'etype' instead of class/type to avoid masking 'type' attribute from Box2d
* Remove unnecessary timestamps from serialization of entities
* Remove 'state' attribute from Entity serialization