Added landing pages for new samples

src/site/samples/appcache/index.markdown

+---
+layout: default
+title: Using Application Cache
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+A simple example to show the use of the Application Cache interface.
+
+For a thorough exploration of this topic, read
+[A Beginner's Guide to Using the Application
+Cache](http://www.html5rocks.com/en/tutorials/appcache/beginner/),
+an article by Eric Bidelman originally published on HTML5Rocks.
+
+Application Cache allows you to specify which files the browser should cache
+and make available to offline users. Your app will
+load and work correctly, even if the user presses the refresh button while
+offline.
+
+You can open the example in Dart Editor and run it by clicking `index.html`.
+Or, you can try this
+[live demo](http://www.dartlang.org/samples/appcache/).
+
+To see this example in action, make changes to `index.html` and then reload
+the app. Your changes don't show up because the browser displays a cached
+version of `index.html`. Next, change the date or version number in
+appcache.mf, and reload once more. You'll be prompted to load the new version.
+
+Read the
+[source](https://github.com/dart-lang/dart-samples/tree/master/web/html5/appcache/beginner).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/).