Runable with SDL2 back-end!

  More: There is no code like `extern crate sdl2;` in the source meaning
  `rust-snake` doesn't depend on `rust-sdl2` directly. It is depended on
  `piston` which requires `rust-sdl`.
Updated `README.md`
Can switch to GLFW back-end with just one line
Merge pull request #136 from Coeuvre/sdl2-back-end

Sdl2 back end