login(username, password){
    return Observable.create(observer => {
      return this.http.post(this.BACKEND_URL + '/login', {username: username, password: password})
      .map(res => res.json())
      // .catch(this.handleError)
      .subscribe(success => {
        // this.data = data;
        observer.next({success: true, data: success});
        observer.complete();
      },
      error => {
        observer.next({success: false, data: error})
      })
    })
  }