moving to markdown

I think this might make it easier to read.

I think this will take a novice more than 45 minutes.  There are plenty of places to get stuck, particularly in terms of entering the code in certain places.

I think critically this tutorial would have a completed working version for reference.  There are some style changes we might also make in the js for the novice, e.g. simpler iterators, expanding ++/-- and some other general js style issues we might address, e.g. placement of var statements ...