Changed  `render` on `Game` trait to take '&mut self'

Closes https://github.com/PistonDevelopers/piston/issues/387
Merge pull request #388 from bvssvni/master

Changed  `render` on `Game` trait to take '&mut self'
Updated example in slides

Closes https://github.com/PistonDevelopers/piston/issues/389
Merge pull request #390 from bvssvni/master

Updated example in slides
Removed PortAudio code

Closes https://github.com/PistonDevelopers/piston/issues/391
Merge pull request #394 from bvssvni/master

Removed PortAudio code
Removed audio code

Closes https://github.com/PistonDevelopers/piston/issues/392
Merge pull request #395 from bvssvni/master

Removed audio code
Added conditional compilation for GLFW and SDL2

Closes https://github.com/PistonDevelopers/piston/issues/396
Merge pull request #397 from bvssvni/master

Added conditional compilation for GLFW and SDL2
Upgraded to latest Rust-Empty
Merge pull request #398 from bvssvni/master

Upgraded to latest Rust-Empty