Rewrite the text of js/lesson3

Substantially the same exercise, but rewritten the instructions to:

 * have fewer words up front, and get started on making things sooner

 * introduce each concept at the time when it's about to be used

 * show code examples only for the first time something new is used

 * walk people through each new thing on the console, then let them
   write the .js file on their own using things they've already seen

Author: asuffield