Re-implement phantom's pagecallback.js example with puppeteer

This patch adds examples/pagecallback.js file which illustrates
phantom's pagecallback example re-implemented with puppeteer API.

examples/pagecallback.js

@@ -0,0 +1,32 @@
+/**
+ * Copyright 2017 Google Inc., PhantomJS Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+var Browser = require('../lib/Browser');
+var browser = new Browser();
+
+browser.newPage().then(async page => {
+    page.on('ConsoleMessage', console.log);
+    await page.setInPageCallback('callPhantom', msg => {
+        console.log("Received by the 'phantom' main context: "+msg);
+        return "Hello there, I'm coming to you from the 'phantom' context instead";
+    });
+    await page.evaluate(function() {
+        // Return-value of the "onCallback" handler arrive here
+        var callbackResponse = window.callPhantom("Hello, I'm coming to you from the 'page' context");
+        console.log("Received by the 'page' context: "+callbackResponse);
+    });
+    browser.close();
+});