feat(core): Add a long-form syntax for Angular bootstrapping.

This change adds a syntax for bootstrapping Angular on a page that allows more fine-grained control of the hierarchy created. platform() creates a platform injector (of which there can only be one). From the platform, .application() creates an Angular application including a Zone and all specified application bindings (e.g. for the DOM, HTTP, Compiler, Renderer, etc). At the application level, .bootstrap() will bootstrap the given component into that application.

Closes #3852

modules/angular2/src/core/application.ts

@@ -1,3 +1,4 @@
 // Public API for Application
-export {ApplicationRef} from './application_ref';
 export {APP_COMPONENT} from './application_tokens';
+export {platform, commonBootstrap as bootstrap} from './application_common';
+export {PlatformRef, ApplicationRef, rootBindings} from './application_ref';