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!
I was thinking the same thing, but I didn't implement it because I wasn't entirely sure about this approach (script loader etc..) and thought it might be beyond the scope of this particular issue.