Adds basic sprite sheet support to sprites

sky/examples/game/main.dart

 import 'dart:sky';
 
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/widget.dart';
+import 'package:sky/framework/net/fetch.dart';
 
 import 'lib/game_demo.dart';
 import 'lib/sprites.dart';
 
 void main() {
   // Load images
   new ImageMap([
-      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/resources-auto/BurnTexture.png",
-      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/asteroid_big_002.png",
-      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/GG_blueship_Lv3.png",
-      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/laserBlue.png",
-      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/laserFlashPurple.png",
-      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Source/Resources/NebulaClouds.png",
+      "res/nebula.png",
+      "res/sprites.png",
     ],
-    allLoaded);
+    allImagesLoaded);
 }
 
-void allLoaded(ImageMap loader) {
+void allImagesLoaded(ImageMap loader) {
   _loader = loader;
+
+  fetchBody("res/sprites.json").then((Response response) {
+    String json = response.bodyAsString();
+    _spriteSheet = new SpriteSheet(_loader["res/sprites.png"], json);
+    allResourcesLoaded();
+  });
+}
+
+void allResourcesLoaded() {
   runApp(new GameDemoApp());
 }
 
 class GameDemoApp extends App {
 
   Widget build() {
     return new Stack([
-      new SpriteWidget(new GameDemoWorld(_loader)),
+      new SpriteWidget(new GameDemoWorld(_loader, _spriteSheet)),
 //      new StackPositionedChild(
 //        new Flex([
 //          new FlexExpandingChild(
 //            new RaisedButton(child:new Text("Hello")),
 //            key: 1
 //          ),
 //          new FlexExpandingChild(
 //            new RaisedButton(child:new Text("Foo!")),
 //            key: 2
 //          )
 //        ]),
 //        right:0.0,
 //        top: 20.0
 //      )
     ]);
   }
 }
 
 ImageMap _loader;
+SpriteSheet _spriteSheet;