* added physics branch: to test timestep fixing (see
http://gafferongames.com/game-physics/fix-your-timestep)
* tested: Semi-fixed timestep
* completed tests on Semi-fixed timestep completed.
* tests on "Free the physics" completed.
* moved physics to external file.
* moved physics to external file. Correction.
Merge branch 'physics'

Conflicts:
	src/main.cpp