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/jsonp/index.ts

@@ -0,0 +1,7 @@
+import {bootstrap} from 'angular2/bootstrap';
+import {JSONP_PROVIDERS} from 'angular2/http';
+import {JsonpCmp} from './jsonp_comp';
+
+export function main() {
+  bootstrap(JsonpCmp, [JSONP_PROVIDERS]);
+}