work in progress..

* most of the "replay" infrastructure is built
* none of it is tested

I actually just tried to run a 1st test, loading the URI:
  moz-rewrite:/print/wget

..but FF threw a "the address wasn't understood" error message.
..this happens BEFORE the addon even gets the opportunity to intercept the request.
..shame, seemed like a clever idea

anyway, all of the library functions are high-level.
this set-back doesn't dimish their value.
I just need to rethink exactly how the user is going to get to the point
where they can view the saved data, and trigger "replay" requests.