Improve Zoom Functionality

[sdellis]

While "zoom" may not be a primary function of the use case for the BookReader, it will be necessary for larger format images such as newspapers and atlases. Implement OpenSeadragon for zoom functionality and reconcile zooming gestures with paging gestures, perhaps through zooming and paging "modes".

[sdellis]

This is another deep zooming option that gets us the Leaflet API for free:
https://github.com/mejackreed/Leaflet-IIIF