Switch to sdl2 and rename main file.
Updated readme to use sdl2 instead of glfw.

This doesn't chage the text because I'm writing a tool to do that
automatically.
Use a markdown preprocessor to keep code examples up to date.

Running `cargo run` will compile the tutorial `.dev.md` file
to a regular `.md` file for viewing.
update for recent piston build
fix rebase mistake
update to new opengl_graphics version
Merge pull request #8 from TyOverby/md-compile

Move the tutorial to sdl2 and automate keeping the tutorial up to date