Add an interception point to allow image URLs to be rewritten

Make all attempts to get a URL for an image go through a single
routine, OpenLayers.Util.getImageLocation, which is given the actual
image name.

This allows users to replace that routine with an enhanced version
which can rewrite the full URL, for example to go through the rails
asset pipeline when using OpenLayers with rails.

lib/OpenLayers/Marker.js

@@ -231,7 +231,7 @@ OpenLayers.Marker = OpenLayers.Class({
  * {<OpenLayers.Icon>} A default OpenLayers.Icon to use for a marker
  */
 OpenLayers.Marker.defaultIcon = function() {
-    var url = OpenLayers.Util.getImagesLocation() + "marker.png";
+    var url = OpenLayers.Util.getImageLocation("marker.png");
     var size = new OpenLayers.Size(21, 25);
     var calculateOffset = function(size) {
                     return new OpenLayers.Pixel(-(size.w/2), -size.h);