fix unboxed demo

* image uris were still referring to thinkfu.  this makes image
  uris relative to location of playground, which requires loosening
  the uri policy to allow relative links.
* "restart" was broken.  model.board is assigned to an object at
  model creation time, which is the same as the local var 'board'.
  model.reset() reassigns local var 'board' to a new object, but
  model.board is still the old object.  so internal to model we
  have a new board, but external users of model still see the old board.

[Jasvir, 2011-03-23 21:36:59]

LGTM

http://codereview.appspot.com/4298056/diff/1/src/com/google/caja/demos/playgr...
File src/com/google/caja/demos/playground/examples/unboxed/index.html (right):

http://codereview.appspot.com/4298056/diff/1/src/com/google/caja/demos/playgr...
src/com/google/caja/demos/playground/examples/unboxed/index.html:86: debugger;
nuke the debugger line too.

http://codereview.appspot.com/4298056/diff/1/src/com/google/caja/demos/playgr...
src/com/google/caja/demos/playground/examples/unboxed/index.html:197: debugger;
ditto