default image for 404s, made Riiif::Image's Riiif::File overridable
Merge pull request #14 from curationexperts/image_uri

added default imgage for 404s, made Riiif::Image's Riiif::File overridable
closed existing viewer on load for turbolinks; closes #15
Merge branch 'turbolinks_bug' of github.com:curationexperts/riiif into turbolinks_bug