AUI-1305 Creating a base class for viewing images

We've noticed that the carousel and the image viewer currently share a lot of behavior. This is refactoring this behavior into a single base class, and having these modules use it.
AUI-1305 Converting the carousel to extend from ImageViewerBase
AUI-1305 Refactoring slideshow behavior into aui-image-viewer-slideshow
AUI-1305 Converting ImageViewer to use ImageViewerSlideshow