Refactor ScreenshotUtils into better classes

[dpulitano]

I think the utils package shouldn't include the screenshot-taking functionality - the methods should be moved to the widget class if the screenshot is for the widget and should be in ExtWebDriver if the screenshot is for the entire browser window.

I propose ScreenshotUtils only has the code to perform comparisons and is renamed to ScreenshotComparator (This could be moved outside of ExtWebDriver at some point)

[dpulitano]

Pull request #87