add iterables article

staged at:
http://iterables.dart-lang.appspot.com/articles/m3-whats-new/iterables.html

also see:
http://iterables.dart-lang.appspot.com/articles/

I removed the puzzlers, since they haven't been updated and were making it harder to see the new article (due to the layout of the articles/ page).

Review URL: https://chromiumcodereview.appspot.com/11896080