Added "count by" block to turtle tutorial.
This was accidentally left out of earlier CLs.

R=root@neil.fraser.name

Review URL: https://codereview.appspot.com/11375043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1094 c400ca83-b69d-9dd7-9705-49c6b8615e23