title: Ionic 2, this.nav.push() is undefined
body: <p>I come from Ionic 1 and I'm starting with ionic 2.
  I get an error when I use this.nav.push(nameOftheOterPage) right after a promise but this function this.nav.push is totally working when I use it outside the promise function authHandler:</p>
  
  <pre><code>    import {Page, NavController} from 'ionic-angular';
      import {ItemDetailsPage} from '../item-details/item-details';
  
      @Page({
        templateUrl: 'build/pages/login/login.html'
      })
      export class LoginPage {
        static get parameters() {
          return [[NavController]];
        }
  
        constructor(nav) {
          this.nav = nav;
          this.firebaseUrl = "https://MYFIREBASE.firebaseio.com";
          this.authType = "login";
          this.error_username = false;
        }
  
        login(form) {
        if(form.controls.username.value === null){
  
          this.error_username = true;
        }
        else{
            /* Authenticate User */ 
            var ref = new Firebase(this.firebaseUrl);
            ref.authWithPassword({
              email    : form.controls.username.value,
              password : form.controls.password.value
            }, this.authHandler);
            }
          }
  
        authHandler(error, authData) {
          if (error) {
              console.log("Login Failed!", error);
          } else {
              console.log(authData);
              this.nav.push(ItemDetailsPage);
          }
        }
      }
  </code></pre>
  
  <p>Could you help me with it?</p>
  
  <p>Thanks.</p>