在Box2D实体界面流畅libGDX创建

Provides syntactic sugar around the libgdx physics api to enable code like this: ``` Body body1 = B2d .staticBody() .withFixture(B2d .box(5, 1) .at(0, -2) .category(1) .mask(2)) .create(world); Body body2 = B2d .dynamicBody() .at(0, 5) .withFixture(B2d .circle() .radius(0.5f) .restitution(0.9f) .category(2) .mask(1)) .create(world); Joint joint = B2d .distanceJoint() .between(body1, body2) .collideConnected(true) .localAnchorA(0, 0) .localAnchorB(0, 0) .length(7) .create(world); ```