new xblock-adaptor embed.html and associated code/images

I'm not entirely sure why the Ext.example code (and stylesheets) that were added to xblock_header.html is in my code... I have a feeling it came from the time I was trying to get popup windows to appear within worldmap.  Perhaps Matt remembers where this code came from.

The rest of this code should solve the JSON.parse(event.data) bug that Steve uncovered yesterday.

The attempt with this code is to make the bare-bones running of embed.html have no bugs so that the same template can be used for worldmap-xblock-adaptor usage as well as bare-bones browser usage (ie: fail softly, silently).