feat(di): Add the @Injectable annotation

Add an annotation marking a class as available to `Injector`s.

modules/angular2/src/di/annotations.js

@@ -107,4 +107,23 @@ export class DependencyAnnotation {
   @CONST()
   constructor() {
   }
-}
+}
+
+/**
+ * A class annotation that marks a class as available to `Injector`s for
+ * creation.
+ *
+ * ```
+ * class NeedsService {
+ *   constructor(svc:UsefulService) {}
+ * }
+ *
+ * @Injectable
+ * class UsefulService {}
+ * ```
+ */
+export class Injectable {
+  @CONST()
+  constructor() {
+  }
+}