Fix for Single mode in ScrollList -  Make it stop at boundaries.

# How to +10/QA
* Run `grunt qa`
* Go to http://0.0.0.0:9000/examples/scroll_list/?scroll=single
  * Verify that clicking and dragging to the top or bottom of the viewport results in a hard stop, instead of a slow boundary (and snapback)
* Go to http://0.0.0.0:9000/examples/scroll_list/?scroll=peek
  * Verify that click-and-drag/swipe still result in a slow boundary at the top and bottom of the list, and allows peeks to the next or previous page.
* Go to http://0.0.0.0:9000/examples/scroll_list/?scroll=flow
  * Verify that the top and bottom of the list still has a slow boundary during click-and-drag/swipe

@timmccall-wf @lancefisher-wf @patkujawa-wf @georgelesica-wf 
Can just one-liner this by `var yBoundaryMode = options.mode === ScrollModes.SINGLE ? 'stop' : 'slow'`.