Allow developers to change the colour palette (issue 149).  Fix Turtle from alerting 'false' on infinite loops (standardize on throwing Infinity).  Remove null block from Turtle.
Review URL: https://codereview.appspot.com/11382043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1096 c400ca83-b69d-9dd7-9705-49c6b8615e23