Add YouTube fallback

Adds Video.js-based YouTube fallback using Khan Academy's YouTube detection method and [videojs](http://www.videojs.com/) HTML5 and SWF video playing.

The current logic:

1. If they can reach [http://youtube.com/favicon.ico](http://youtube.com/favicon.ico), uses standard YouTube player
  * This will eventually point to youtubeeducation or ytimg instead
2. If they support Flash player, uses video.js Flash player fallback (.swf hosted on our site)
3. If they support HTML5 `video/mp4` video playback, uses standard HTML5 player ([videojs](http://www.videojs.com/) styled with centered play button)
4. If they don't support any of these, displays default video.js "Download Chrome or Flash" message

![image](https://cloud.githubusercontent.com/assets/206973/2711680/7218078a-c4d0-11e3-875d-bd3b7b0715ae.png)

![image](https://cloud.githubusercontent.com/assets/206973/2711687/7d457ad4-c4d0-11e3-9b25-f24c4335de9f.png)

![image](https://cloud.githubusercontent.com/assets/206973/2711678/6d46642c-c4d0-11e3-96c7-be8dc185c5d3.png)