* 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
