This is a proof-of-concept for using the post-to-iframe method of setting
cookies for x-domain iframe requests. Facebook uses this method for iframe
canvas page apps.

See the images in screenshots/ to see how this works in browesrs.

Basically what happens is we request a page on the iframe-widget-test-base
domain which generates a random token. Then with JS on page load, we submit
a form that has the iframe as the target. The form action url is, of course,
a separate domain: iframe-widget-test-frame. In that request to the frame we
set a cookie in the response to the randomly generated token. We then use JS in
that frame to show that the cookie is indeed set for the frame's domain. As
a control, we load another iframe (on the right in the pictures) with a normal
GET request (iframe's src attribtue) on the same domain as the base page
(iframe-widget-test-base) to show that no cookie gets set for the base page.
Finally, to show that the x-domain iframe is indeed isolated, we try to use JS
in the base page to access the title of the frame. In the x-domain iframe, we
cannot access the title (browser security enforces that JS cannot communicate
between frames with different domains). In the same-domain iframe, we can
access the title.

To run this yourself...

This test requires entries in /etc/hosts; e.g.:

    127.0.0.1 iframe-widget-test-base.urtak.com
    127.0.0.1 iframe-widget-test-1.urtak.is
    127.0.0.1 iframe-widget-test-2.urtak.is
    127.0.0.1 iframe-widget-test-3.urtak.is
    127.0.0.1 iframe-widget-test-4.urtak.is

Run with shotgun: `bundle exec shotgun`.

**Also, somehow, you'll need to forward requests to port 80 to 9393 (for use
with shotgun).**

Navigate to http://iframe-widget-test-base.urtak.is/ in your browser.

Currently the post-to-iframe technique works in:
- FF3+
- IE6+
- Chrome (every version I've tried...)
- Opera (9+ I think...)
- Safari (before v6)
- mobile Safari (before iOS v6)

Safari 6+ and mobile Safari in iOS 6+ threw a wrench in the whole technique
though. It looks like localStorage works in cross-domain iframes, though, so
that may provide a work around for the future (requires application
rearchitecting).

Chrome with "block third party cookies and site data" does not even allow
localStorage.