Hook into WebDriver

[ianb]

WebDriver is more "accurate" for interacting with the browser than we can be in-content. Walkabout.js could still detect plausible events, but instead of actually performing them it could send them to some parent WebDriver process.

[ianb]

Note some high-level events might still be easier to do in content (e.g., we might be able to trigger a swipe directly in content, but it may be composed of several underlying events).