Plan on Physics Simulation Service

Implementation choices:
1. chipmunk-physics
2. box2d

Requirement of physics simulation service: (draft)
1. create a world
2. create a ground body
3. create a dynamic body
4. simulate the world
5. apply a force
6. change the gravity of the world

Plan: Re-factor Testbed project of Box2D to distill the interface of physics simulation service.

Motivation: I was inspired by Martin, Clean Code. I need physics simulation, and I know that using third-party code is a good idea. Don't reinvent the wheel. So I need to explore and learn boundaries of physics simulation. So I have a plan.

Besides, I can use code that does not yet exist. If I have any trouble in implementing the service, I can provide a fake implementation to ensure that other parts can be tested and worked.