fix(auth): make sure onAuth runs in Angular zone

There is one place in auth that takes a promise from Firebase,
the promise from getRedirectResult(), and creates an observable
from it. Since Firebase is using their own Promise implementation,
the Promise and its derived observable were running in the root 
zone instead of the Angular zone, causing onAuth changes to not
immediately be propagated to the view. By casting the firebase
Promise to a zone-patched Promise, the observable now runs
in the Angular zone.

Fixes angular#231

src/providers/firebase_sdk_auth_backend.ts

@@ -94,7 +94,7 @@ export class FirebaseSdkAuthBackend extends AuthBackend {
   }
 
   getRedirectResult(): Observable<firebase.auth.UserCredential> {
-    return Observable.fromPromise(<Promise<any>>this._fbAuth.getRedirectResult());
+    return Observable.fromPromise(<Promise<any>>Promise.resolve(this._fbAuth.getRedirectResult()));
   }
 
   private _enumToAuthProvider(providerId: AuthProviders): firebase.auth.AuthProvider | FirebaseOAuthProvider {