Update main.cpp

implemented physics part
Create Physics.h
Create Physics.cpp
Create test03.txt
Update SpaceObject.h
Update SpaceObject.cpp
Merge pull request #1 from kangtianyu/patch-1

Update main.cpp
Merge pull request #2 from kangtianyu/patch-2

Create Physics.h
Merge pull request #3 from kangtianyu/master

a runable version of gravity sim with testfile