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!
Rather than the if statement here can we do something like 

```
<%= render_viewer_partial %>
```
then a helper like this
```
# in app/helpers/geoblacklight_helper.rb, would also require renaming and new location of viewer partials
def render_viewer_partial
  begin
    render "catalog/viewers/_viewer_#{document.preferred_viewer_type}"
  rescue ActionView::MissingTemplate
  end
end