Added support for postMessage's into any iframes for content that needs ...

...to do something when a slide is displayed and hidden.

Second pull request attempt for issue https://github.com/hakimel/reveal.js/issues/661 (addresses rejection issues for pull request https://github.com/hakimel/reveal.js/pull/675)

This patch ignores the `data-ignore` attr for the stop event and targets the dev branch.

Assuming this gets merged, how do I know when this puppy gets deployed into production in slid.es? When it hits master?

Thanks Hakim and cheers!