Created common.js for code common to multiple apps.
Review URL: https://codereview.appspot.com/7808046

git-svn-id: http://blockly.googlecode.com/svn/trunk@722 c400ca83-b69d-9dd7-9705-49c6b8615e23
Add first two levels of turtle tutorial.  This does not check users'
answers or advance through levels.
Review URL: https://codereview.appspot.com/7741051

git-svn-id: http://blockly.googlecode.com/svn/trunk@723 c400ca83-b69d-9dd7-9705-49c6b8615e23