Make sure we're using the window context when evaling requirejs scripts

require-cs.js is a must have for using requirejs with content_scripts. I had an issue using it with underscore though because underscore defines itself on "this":

```
//     Underscore.js 1.5.1
//     http://underscorejs.org
//     (c) 2009-2013 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
//     Underscore may be freely distributed under the MIT license.

(function() {

  // Baseline setup
  // --------------

  // Establish the root object, `window` in the browser, or `global` on the server.
  var root = this;
```

as is, this would be the xhr request, using "call" and passing in "window" fixes the issue.