Add yt-toggle for Polycast embeds

Adds a check for any elements with a `.yt-toggle` class and swaps their content with an iframe video player when the user clicks. The `.yt-toggle` element must have a `data-yt` attribute with the id of the YouTube video you wish to embed.

I'm still waiting on the thumbnail from GDS so I used one of Eric's devbytes to demo. I'll update the PR with the final videos and thumbnails when they become available.

@ebidel @addyosmani PTAL
How about http://www.polymer-project.org/images/logos/polymer_video_thumb.jpg instead?