Sandboxing: Adding disclaimers about `postMessage` + cleanup.

1.  `postMessage` can be dangerous if you're not careful; the demo code in
    this article didn't make that clear. This patch adds some comments to
    ensure that the important points are clearly highlighted, and adjusts
    the demo code itself to actually do rudimentary validation of sources
    rather than blindly executing code in response to a message event.

2.  Cleanup in the formatting of some code at the bottom of the article.
    One space too many, ah well.
@abarth's feedback.

* e.origin === sandboxedFrame.contentWindow
Update content/tutorials/security/sandboxed-iframes/en/index.html

e.origin === frame.contentWindow.
Update content/tutorials/security/sandboxed-iframes/en/index.markdown

e.source === frame.contentWindow
Merge pull request #263 from mikewest/sandboxfixup

Sandboxing: Adding disclaimers about `postMessage` + cleanup.