Improve Sky READMEs

sky/examples/hello_world/hello_world.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "sky/compositor/rasterizer.h"
+import 'package:sky/framework/fn.dart';
 
-namespace sky {
-
-Rasterizer::Rasterizer() {
+class HelloWorldApp extends App {
+  Node build() {
+    return new Text('Hello, world!');
+  }
 }
-
-Rasterizer::~Rasterizer() {
-}
-
-}  // namespace sky