Enhancement/osd for all img 56

[mnyrop]

Closes #56

• Converts simple_image_viewer.html to use openseadragon with direct full image tile source
• Gets rid of the need to specify image_viewer: openseadragon vs image_viewer: simple in page or layout; instead generic_collection_item.html will:
  • use osd_iiif_image_viewer.html if page.manifest exists
  • use simple_image_viewer.html if page.full exists but page.manifest does not
  • show no viewer by default if neither page.manifest nor page.full are set
• Adds functionality for simple_image_viewer.html to render items with multiple, paged assets via img_list variable, but this feature NEEDS to be implemented in wax_tasks (see: wax_tasks#90) for that info to be propagated to the collection page yaml