Added hook for specifying HttpHeaders

Needed to be able to specify a CSRF token in my app when using EmberDroplet so I came up with this code. Please let me know your thoughts.  Now I can do the following:

````javascript
App.FilesUploadController = Ember.Controller.extend(DropletController, {

     requestHeaders: {
        'X-CSRFToken': ENV.CSRF_TOKEN
     }

});
````