graphics -> production

- Fixed sprint 1 integration test issues: The test now runs successfully (though grass.png still needs to be added, and tank.png needs to be updated)
- Refactored creation of entities a bit: merged <code>Entities.create</code> and <code>World.addEntity</code> into one method. To add entities and sprites to the world, call <code>World.addEntity</code>.
- Implemented much of the network system, excluding the <code>NetworkCommand</code>s
- Greatly increased the number of unit tests
- Resolved merge conflicts between the Sprint1Integration branch and Graphics branch
- Updated the game loop: Rendering is now restricted to 60 fps, while logic updates continue to run at 30 fps