add IIIF vocab

[ndushay]

This is the Intl Image Interop Framework (IIIF) Presentation API vocab -

• spec: http://iiif.io/api/presentation/2.0/
• ontology: http://iiif.io/api/presentation/2/ontology.xml

If this is okay, it would be awesome if a gem release could be cut with this. Thanks!