closed #5537: add setAutoStep, isAutoStep and step method for PhysicsWorld.
closed #5537: add test case
Merge branch 'v3' of https://github.com/cocos2d/cocos2d-x into feature5537_physics_step

Conflicts:
	tests/cpp-tests/Classes/PhysicsTest/PhysicsTest.cpp
	tests/cpp-tests/Classes/PhysicsTest/PhysicsTest.h
change NULL->nullptr
fix some merge errors.
Merge pull request #7037 from boyu0/feature5537_physics_step

closed #5537: add setAutoStep, isAutoStep and step method for PhysicsWorld