support iiif level 1
Only require IIIF level 1; Vary tile size based on src image when no tiles are reported.  Toward openseadragon/openseadragon#313
style changes, per @iangilman on openseadragon/openseadragon#315
Merge pull request #315 from IIIF/support_iiif_level1

Support iiif level1
Changelog for #315