Add Wheel to Webdriver Actions API

Hello all,

We have briefly talked about supporting wheel scroll actions in Webdriver
in last year's TPAC seminars. I have summarized our discusses and proposed
some possible ways of exposing it to the Actions API, below is our design
doc on adding the wheel scroll actions with some mouse wheel tests, please
take a look
https://docs.google.com/document/d/1AzPkvexzuUJ1SF-6pZFpnn2lwEYlk3T0e-Wi52UWFZ4/
..

Please feel free to comment on the design doc, thank you.

Lan Wei