Differentiate between HTML and XML artifacts; add Appium screenshot tests

In this release, I added differentiation between HTML artifacts and XML artifacts to clarify diagnostic content. If page source capture is blocked by an unhandled alert, I now package up the alert text in a mini HTML page artifact. The screenshot capture implementation now gives drivers the benefit of the doubt when the ability to take a screenshot cannot be conclusively determined. I also added a dispose() call to the Graphics object created to produce the screenshot proxy image for scenarios where screenshot capture fails.

To exercise the newly discovered ability to capture Appium screenshots, I added unit tests for Android, iOS, Mac, and Windows.