Add ol.source.ImageArcGISRest (#3880)

* Add ol.source.ImageArcGISRest

ol.source.ImageArcGISRest is added so a single image can be retrieved
which is required when labelling is applied to the underlying map
service.
it's based on the approach used in ol.source.ImageWMS

* updating to remove as many good references as possible
changing so js passes lint checks

* fixing up so build and tests run

* putting back what was lost in the merge

* adding in an example
tidying up

examples/arcgis-image.html

@@ -0,0 +1,11 @@
+---
+layout: example.html
+title: Image ArcGIS MapServer
+shortdesc: Example of an image ArcGIS layer.
+docs: >
+  This example shows how to use a dynamic ArcGIS REST MapService.
+  This source type supports Map and Image Services. For dyamic ArcGIS
+  services.
+tags: arcgis, image, dynamiclayer"
+---
+<div id="map" class="map"></div>