Option to disable drag scrolling

This adds the `touchScrollingEnabled` option for `ScrollList`. When enabled, you can scroll and pan with dragging and swiping. When disabled, you can't.

How To +10/QA
-------------

```bash
git fetch && git checkout option_to_disable_drag_scrolling
./init.sh && grunt qa
#All tests should pass.
grunt serve
```

* Open [http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=true](http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=true)
* You should be able to drag to scroll the document.
* Now, open [http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=false](http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=false)
* When you drag, nothing happens.

Currently _every event_ that is detected by the AwesomeMap ends up causing `onInteraction` to be dispatched. This gives observers the opportunity to know about everything that might happen. I think for general usability, we want to continue to allow this to happen. 

If you look at line 648 below, you'll see the conditions where the event is not transformed. Would be best to include this logic in that set of conditions. As the conditions are getting long at this point (this will be the third scenario that prevents event transformation), might want to put in a private `_isTransformationCanceled(event)` method or something.