adds iiif image viewer

This is a fist attempt at implementing an OpenSeaDragon IIIF image viewer as well adding functionality to switch between viewer types. 

A few notes:

- The javascript modules, in general, need some throughtful updating and cleanup. I sidestepped that work for the most part here to get this out quick.
- There is an OpenSeaDragon ruby gem, but I had difficulty in getting it to work. I think it would be a good idea to revisit this at some point. Also, in order to support IIIF v.2, OpenSeaDragon must be compiled from source.

Eager for feedback on this!
This would allow us to add new viewer partials and via `References` class rather than having to modify this if/else block every time.