Embeded video

This introduces external embed functionality to our media player for video content. The button is only shown on content we have rights to embed and is behind a feature switch for performance reasons.

It also introduces the external endpoint which is iframed from 3rd party sites and therefore requires its own JS bootstrap and external CSS file. The endpoint is hosted and CDN'd via our new secure domain https://embed.theguardian.com/embed/video

- New route for embeds on application server
- New video-embed.js standalone application
- New media-player.css standalone CSS
- Open-sourced [video-embed](http://github.com/guardian/videojs-embed) plugin for videojs player

### Screenshot
![google chrome 29](https://cloud.githubusercontent.com/assets/80089/5300673/05defb44-7bca-11e4-9b78-5017569ec76f.jpg)

### Todo
- Tests for endpoint
- Monitoring for endpoint
- Test on a new external sites before switching on 100%

/cc 
@rich-nguyen @mattosborn @ironsidevsquincy 

some duplication here (`isModernBrowser`) - anything we can share