Support IIIF servers that don't report tile dimensions

`tile_width` / `tile_height` aren't required by IIIF. Some images may not have embedded tiles but will still support region extraction and scaling. Solution is to use the short dimension of the image as the tile size when tile dimensions are not reported.

Also a tiny fix: there is no IIIF level 3 compliance, so took that URI out @ [line 86](https://github.com/jpstroop/openseadragon/compare/openseadragon:master...jpstroop:master#diff-e2c7f0819c3bcbfb0c9a3e3f73166de2L86)

Test and test images included.