Use local storage to save and restore current work when user leave or refresh current page about blockly HOT 4 CLOSED

This looks like a promising addition.  The catch is how does a user clear the 
workspace?  Currently you just hit reload.  But with this code the blocks would 
follow you.  This would be unfortunate if you hit the 'airstrike' button on the 
playground a few times.  :)

I think that in addition to this feature, we need a clear-all UI element.  What 
would that look like?  Each of the three demos look pretty different.

Maybe the best UI for the demos would be to pop open a window.confirm on page 
load asking "Restore saved blocks?"  Ok would restore, cancel would delete the 
local storage.  That way we wouldn't need a clear-all UI element.

As for jquery, I'd prefer to eventually use Closure instead since it works much 
better with the Closure compiler, type checker, Google's build system, etc.  
But in this case you can just call Blockly.bindEvent_(window, 'unload', null, 
backup_blocks)

How about add a 'clean' button after 'run' button? 

Every time user will get their last blocks without worry.
Once user want to clear the blocks, they can do it explicitly on screen.

commited in r242