refactor(LifecycleEvent): remove LifecycleEvent

fixes #3924

BREAKING CHANGE

The `lifecycle` configuration for directive has been dropped.

Before

    // Dart
    @Component({lifecycle: const [LifecycleEvent.OnChanges], ...})
    class MyComponent implements OnChanges {
      void onChanges() {...}
    }

    // Typescript
    @Component({lifecycle: [LifecycleEvent.OnChanges], ...})
    class MyComponent implements OnChanges {
      onChanges(): void {...}
    }

    // ES5
    var MyComponent = ng.
    Component({lifecycle: [LifecycleEvent.OnChanges], ...}).
    Class({
      onChanges: function() {...}
    });

After

    // Dart
    @Component({...})
    class MyComponent implements OnChanges {
      void onChanges() {...}
    }

    // Typescript
    @Component({...})
    class MyComponent implements OnChanges {
      onChanges(): void {...}
    }

    // ES5
    var MyComponent = ng
      .Component({...})
      .Class({
        onChanges: function() {
        }
      });

modules/angular2/metadata.ts

@@ -14,7 +14,6 @@ export {
   ComponentMetadata,
   DirectiveMetadata,
   PipeMetadata,
-  LifecycleEvent,
   ViewMetadata,
   ViewEncapsulation,
   QueryMetadata,
@@ -50,6 +49,8 @@ export {
 } from './src/core/metadata';
 
 export {
+  // todo(vbe): LifecycleHook should not be exposed (fails test.typings)
+  LifecycleHook,
   AfterContentInit,
   AfterContentChecked,
   AfterViewInit,