chore: rename modules/examples to modules/playground

The directory contains code authored in a style that makes it transpilable to dart. As such, these are not idiomatic examples of Angular 2 usage.

The main purpose of this directory is to enable experimentation with Angular within the angular/angular repository.

Closes #4342

Closes #4639

modules/playground/src/gestures/template.html

@@ -0,0 +1,15 @@
+<style type="text/css">
+  .row {
+    height: 33%;
+    text-align: center;
+    border-bottom: 1px solid black;
+    margin: 0;
+    padding: 0;
+    -webkit-user-select: none;
+  }
+</style>
+
+<div class="row" (swipe)="onSwipe($event)">Swipe (direction = {{swipeDirection}})</div>
+<div class="row" (pinch)="onPinch($event)">pinch (scale = {{pinchScale}})</div>
+<div class="row" (rotate)="onRotate($event)">Rotate (angle = {{rotateAngle}})</div>
+<div class="row"></div>