Change helpUrl from property to setter method.  Backwards compatible, setting the property still works.  Using the settor allows this property to be set in the init, rather than on script load, thereby removing requirement that language files load before block files.  Likewise, set dropdown options in init, rather tan on script load.

R=spertus@google.com

Review URL: https://codereview.appspot.com/13613044

git-svn-id: http://blockly.googlecode.com/svn/trunk@1346 c400ca83-b69d-9dd7-9705-49c6b8615e23