issue #2771: finish OneWayPlatform demo and fix some bugs
issue #2771: delay load
issue #2771: enhance API and add Slice demo
issue #2771: change some coding style
issue #2771: fix physics test compile error
issue #2771: move header definition to first line
issue #2771: make api more friendly
issue #2771: fix body and joint memory bugs. add removeFormWorld and destroy to PhysicsJoint, and add a destroy param to PhysicsWorld->removeJoint()
issue #2771: fix PhysicsWorld some bugs
issue #2771: fix PhysicsWorld functions name readadd/removexxx -> doadd/removexxx  delayTestAdd/Removexxx -> add/removexxxOrDelay
issue #2771: change rayCast and rectQuery callback class to std::function
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into iss2771_physical
issue #2771: change PhysicsJoint::create() to PhysicsJoint::construct()
issue #2771: delete some unfinished PhysicsTest and change subtitle
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into iss2771_physical
issue #2771: fix build error
Merge pull request #4111 from boyu0/iss2771_physical

[ci skip]issue #2771: physical