give Tile a unique id. add tests

git-svn-id: http://svn.openlayers.org/trunk/openlayers@980 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Tile.js

@@ -13,6 +13,9 @@
 OpenLayers.Tile = Class.create();
 OpenLayers.Tile.prototype = {
     
+    /** @type String */
+    id: null,
+    
     /** @type OpenLayers.Layer */
     layer: null,
     
@@ -45,6 +48,9 @@ OpenLayers.Tile.prototype = {
             this.bounds = bounds;
             this.url = url;
             this.size = size;
+
+            //give the tile a unique id based on its BBOX.
+            this.id = OpenLayers.Util.createUniqueID(bounds.toBBOX());
         }
     },