Add IIIF Image API tilesource with example

src/ol/source/IIIF.js contains a tile source for IIIF Image API services.
It supports Image API version 1 and 2 on compliance levels 0, 1 and 2.

To get working constructor options for IIIF from an IIIF image info.json,
use src/ol/format/IIIFInfo.js.

An example is available in examples/iiif.html respectivly examples/iiif.js.

src/ol/source.js

@@ -5,6 +5,7 @@
 export {default as BingMaps} from './source/BingMaps.js';
 export {default as CartoDB} from './source/CartoDB.js';
 export {default as Cluster} from './source/Cluster.js';
+export {default as IIIF} from './source/IIIF.js';
 export {default as Image} from './source/Image.js';
 export {default as ImageArcGISRest} from './source/ImageArcGISRest.js';
 export {default as ImageCanvas} from './source/ImageCanvas.js';