Add ability to do screenshot sequences (rough video)

The following public api methods have been added:

 - Solo.startScreenshotSequence(String name)
 - Solo.startScreenshotSequence(String name, int quality, int frameDelay, int maxFrames)
 - Solo.stopScreenshotSequence()

In addition, the ScreenshotTaker now saves screenshots in a background thread, after taking them on the ui thread.  This was done for performance reasons.