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.

Also, when performing simulated panning operations, the following events are generated: `touch > drag > release`. If you're using the ScrollList and jump to a page, the ScrollList will trigger a simulated pan. You can detect these simulated events by checking `args.event.simulated`. If the event is generated by a direct user interaction, this value will be false, and this is the condition in which you'll want to bail.

To see this in action, open http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=false as per the QA instructions, go to the console, and enter `scrollList.scrollTo({ index: 3 })`. Nothing will happen. We want the page to jump on this API call.