Fix undefined root error.

I'm getting this error on backbone 1.0 and backgrid's master:

![screen shot 2013-07-10 at 3 29 55 pm](https://f.cloud.github.com/assets/51424/774869/f9d446cc-e964-11e2-8eb6-66cb4fe543b6.png)

This PR sets the `root` variable to the `window` object.