add an ol.TileCache and use it in ol.TileLayer

src/ol/Tile.js

@@ -88,6 +88,13 @@ ol.Tile.prototype.handleImageError = function(evt) {
     this.events_.triggerEvent('error');
 };
 
+/**
+ *
+ */
+ol.Tile.prototype.destroy = function() {
+    this.events_.triggerEvent('destroy');
+};
+
 /**
  * Create an image node. This is done by cloning
  * the same image element.