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.js

@@ -0,0 +1,29 @@
+goog.require('ol.Map');
+goog.require('ol.View');
+goog.require('ol.layer.Tile');
+goog.require('ol.layer.Image');
+goog.require('ol.source.MapQuest');
+goog.require('ol.source.ImageArcGISRest');
+
+var url = 'http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/' +
+    'Specialty/ESRI_StateCityHighway_USA/MapServer';
+
+var layers = [
+  new ol.layer.Tile({
+    source: new ol.source.MapQuest({layer: 'sat'})
+  }),
+  new ol.layer.Image({
+    source: new ol.source.ImageArcGISRest({
+      params: {},
+      url: url
+    })
+  })
+];
+var map = new ol.Map({
+  layers: layers,
+  target: 'map',
+  view: new ol.View({
+    center: [-10997148, 4569099],
+    zoom: 4
+  })
+});