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!
Do you think we could get away with two separate submodules `Geoblacklight.Item.Iiif` and `Geoblacklight.Item.Wms` that lived in different js files? That way both could potentially be used on the same page somewhere? This could remove this logic... and just instantiate based off of the different supplied data-attributes.  @gravesm do you feel strongly about any of this?