[plt-scheme] Danny's tutorial

> You might use some of the Scheme Cookbook content as sample data while
> prototyping.  (Consideration of copyright issues wrt the Scheme Cookbook
> content can be deferred until the system is ready to go live, I suspect.
> Any issues might be resolved trivially once there's a new system people
> can look at.)

Let's just do it.  *grin*

     http://schemecookbook.org/Cookbook/IntroductionToMzlibClasses

I'll continue work on this, and use GettingStartedRecipes at the 
ParentTopic for now.  Gotta fix a few twiki-ish things, but let me edit
Ryan's suggestions in first before I make it pretty.