"platform" collision objects not fully covered in tutorial

[trevorjwilliams]

There are a few areas for improvement with the handling of "platform"s in the tutorial:

1. "down" is never bound game.js in the tutorial, so the fall-through logic doesn't work.
2. The section on adding the "platform" type to collision boxes might be better if it were explicitly integrated in the Tiled section beforehand since that's where the change needs to be made.