New Tutorial and core.typed fixes

I have transferred and updated an article I wrote for a PragPub 2011 volume. I have confirmed with PragPub that the article is indeed mine to do with as I wish.

It was originally published here: http://pragprog.com/magazines/2011-07/growing-a-dsl-with-clojure

It needed a new home because it had a long errata and PragPub could not update the article for me.

I have also renamed the core.typed files to be lower-case and replaced spaces with underscores.