Support iiif level1

This addresses issue #313:

 * Only IIIF level 1 compliance is required (use `w,` syntax for the size instead of `w,h`)
 * If the server does not report a tile size for the image, a size is determined based on the size of the image.
 * OpenSeadragon determines that the image metadata is from an IIIF server based on the presence and value of `data['@context']`

Tests are added and/or revised to suit these new behaviors.