Implement http://snake-tutorial.zeespencer.com/ in stepfile format

Offered as somewhat of a counter-proposal to #347. I tried to include the content from http://snake-tutorial.zeespencer.com/ as its own site directly as HTML, but hadn't realized the extra cleverness it was employing rendering an _expected_result on each page all jekyll-like. So I decided to see how much effort it would be to just do it as a series of .step files.

Known issues:
* Page titles aren't really present (other than 'lesson 1', 'lesson 2'). Doing this exercise is making me feel more like divorcing page titles from filenames a little bit.
* js_expected_results function is currently in step.rb but should be able to live in some site-specific mixin that's only available for the JS Snake Game site
* I tried something dumb and clever for automatically zipping things, which works locally but not yet on Heroku

Deployed for review at http://docs-javascript-snake.herokuapp.com/javascript-snake-game

Thoughts? Comments? Style questions? Code questions? Should this exist? Should this not exist? What is real?

Paging @zspencer @alexch @nuclearsandwich and anyone else who feels like saying words