Preroll countdown

Adds a permanent label and countdown overlaid on top of preroll adverts within the video player.

- Fix grunt watch task for Sass compilation
- Reset dfpRoot to test-theguardian.com (this is to enable our test preroll campaigns)
- JS logic and styling for countdown

#### Screenshots (ignore the fonts)
![google chrome](https://cloud.githubusercontent.com/assets/80089/3327283/a8c91a6c-f7ac-11e3-9698-e6faac4e8957.png)

![littlesnapper](https://cloud.githubusercontent.com/assets/80089/3327287/ad3723aa-f7ac-11e3-8109-ea70cba20233.png)