refactor: export core APIs from angular2/core

This change moves many APIs to the angular2/core export.

This change also automatically adds FORM_BINDINGS in
the application root injector.

BREAKING CHANGE:
    Many dependencies that were previously exported from specific
    APIs are now exported from angular2/core. Affected exports, which
    should now be included from angular2/core include:

    angular2/forms
    angular2/di
    angular2/directives
    angular2/change_detection
    angular2/bootstrap (except for dart users)
    angular2/render
    angular2/metadata
    angular2/debug
    angular2/pipes
Closes #3977

modules/angular2/angular2.dart

@@ -5,4 +5,4 @@ library angular2;
  *
  * This library does not include `bootstrap`. Import `bootstrap.dart` instead.
  */
-export 'package:angular2/angular2_exports.dart';
+export 'package:angular2/core.dart';