Feat/box2d

[vicksonzero]

• Adds Box2d physics engine in place of existing Phaser-MatterJS.
• Detaches physics from front-end engine
• Makes easier to turn game into headless or resimulate game state in the future.

This version is not perfect. Bugs happen, but game is working.

• PhysicsSystem class
  • constructed outside phaser code,
  • is passed to phaser instead of created by phaser scenes
• World driven by scene update(). Almost does fixed timestep except for small things
• Known bugs
  • Just like the previous versions, collision events are still not consistent -- i added tons of lifecycle logs though. You can use _Debug.enable('tank-beyond-repair:*') in console to enable logs

Resolves #55

[williamchong]

Why change from matters to box2d? @vicksonzero

[vicksonzero]

@williamchong007 discussion here: https://github.com/vicksonzero/tank-beyond-repair/issues/55

[vicksonzero]

@williamchong007 @199911 @skmisaac If no one is reviewing this PR, then i ll just merge it, since all my following commits are based on this branch too