Adds basic support for images in game example

sky/examples/game/lib/image_map.dart

+part of sprites;
+
+typedef void ImageMapCallback(ImageMap preloader);
+
+class ImageMap {
+
+  Map<String, Image> _images;
+
+  int _totalNumImages = 0;
+  int _numLoadedImages = 0;
+
+  ImageMapCallback _callback;
+
+  ImageMap(List<String> urls, ImageMapCallback callback) {
+    _images = new Map();
+    _totalNumImages = urls.length;
+    _callback = callback;
+    urls.forEach(_addURL);
+  }
+
+  void _addURL(String url) {
+    image_cache.load(url, (Image image) {
+      // Store reference to image
+      _images[url] = image;
+
+      // Check if all images are loaded
+      _numLoadedImages++;
+      if (_numLoadedImages==_totalNumImages) {
+        // Everything loaded, make callback
+        _callback(this);
+      }
+    });
+  }
+
+  Image getImage(String url) => _images[url];
+
+  Image operator [](String url) => _images[url];
+}