feat(examples): adds hello-world app.

The app is writen in ES6 and transpiles to ES5 and dart as part of the
usual build.

The app contains a component, a directive and a services wired together
through dependency injection.

Before Each:
- gulp build

For es5:
- gulp serve
- open 'localhost:8000/js/examples/lib/hello_world/'

For dart:
- gulp examples/pub.serve
- open 'localhost:8080'

modules/core/src/core.js

@@ -2,6 +2,7 @@
  * Define public API for Angular here
  */
 export * from './annotations/directive';
+export * from './annotations/decorator';
 export * from './annotations/component';
 export * from './annotations/template_config';
 
@@ -15,3 +16,4 @@ export * from './compiler/compiler';
 export * from './compiler/template_loader';
 export * from './compiler/view';
 
+export * from 'core/dom/element';