Merge pull request #543 from bvssvni/master

Added get_draw_size
Merge pull request #545 from bvssvni/master

Added Retina support
Merge pull request #548 from bvssvni/master

Added 3D camera
Merge pull request #550 from bvssvni/master

Added FPS controller
Removed `ConcurrentGame` and `Game`

Closes https://github.com/PistonDevelopers/piston/issues/555

For fixing existing code, see
https://github.com/PistonDevelopers/piston/issues/521