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!
Yeah, this seems like a good idea. In fact, we might even want to create a modules/viewers/ directory. What would be nice is to have a configurable map of Geoblacklight::Constants::URI keys to viewers. Maybe we could have a GeoBlacklight.Viewer factory that created viewers, so you would do something like:
```javascript
var viewer = GeoBlacklight.Viewer.create('wms');
```
Thinking out loud here.