feat(decorators): adds decorator versions of DI annotations.

In 'angular2/di' the symbol:
- Inject is a decorator
- InjectAnnotation is an annotation

Internally one an get a hold of annotations without *Annotations appened
(to make ts2dart work without workarounds) by importing from
'angular2/src/di/annotations_impl' instead of 'angular2/di'. This is
needed only for users that transpile through TS and through ts2dart.

modules/angular2/annotations.js

@@ -9,4 +9,4 @@
  *
  */
 export * from './src/core/annotations/annotations';
-export * from './src/core/decorators/decorators';
+export * from './src/core/annotations/decorators';