Adding robots.txt to reduce log 404 spam.  Add 'one top block' dialog for Maze.  Move dialog CSS to common template.

R=spertus@google.com

Review URL: https://codereview.appspot.com/12805044

git-svn-id: http://blockly.googlecode.com/svn/trunk@1206 c400ca83-b69d-9dd7-9705-49c6b8615e23