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
The early work. Next commit should be rolling all the articles over to using the new Liquid tag.
Replaced all the tags with the new prettify tags. Also updated the Prettify tag handler to support no language specified.
Ripping out Pygments.
Changing the prettify styles to match what was used with Pygments
Updating for changes that have happened since I last checked out.
Fixing Prettify Liquid tag example usage typo.
Fixing numbers to use the old color value.