Extracted home page IIIF demo to jQuery plugin

Closes #254 

* Updated home page with new IIIF demo jQuery plugin
* The plugin's repo is at [https://github.com/sul-dlss/jquery-iiif-demo](https://github.com/sul-dlss/jquery-iiif-demo) and its demo page at [http://sul-dlss.github.io/jquery-iiif-demo/](http://sul-dlss.github.io/jquery-iiif-demo/)