Handle Mouse Wheel Start and End Events

Handle these events in the EventSynthesizer and publish
`onInteractionStarted` on mouse wheel start events and
`onInteractionFinished` on mouse wheel end events. This allows
consumers to have consistent experience through those events whether
on touch or desktop.

To test:
- bower link the code in `https://github.com/WebFilings/wf-common/pull/4`, 
- open the ScrollList example with the console open,
- as you mousewheel, witness the timing of interaction started/finished logs.