title: AngularFire authWithPassword not working with angular factory
body: <pre><code>  app.factory("Auth", ["$firebaseAuth", function($firebaseAuth) {
    var ref = new Firebase("https://XXXXXX.firebaseio.com/");
    var auth = $firebaseAuth(ref);
  
    var Auth = {
      login: function(){
        return auth.$authWithPassword({
          "email": "1@1.com", //hard-coding in the email &amp; password for testing
          "password": "1"
        }, function(error, authData) {
          if (error) {
            console.log("Login Failed!", error);
          } else {
            console.log("Login Success!", authData);
          }
        });
      }
    return Auth;
  }]);
  
  app.controller("SampleCtrl", ["$scope", "Auth", "$firebaseAuth", function($scope, Auth, $firebaseAuth) {
    $scope.login = Auth.login();
  }]);
  </code></pre>
  
  <p>I am trying to create a factory to use the angularFire user authentication function</p>
  
  <pre><code>&lt;button ng-click="login()"&gt;Login&lt;/button&gt;
  </code></pre>
  
  <p>It returns an error: <code>TypeError: Cannot read property '0' of null</code> 
  What am I doing wrong?
  <a href="http://plnkr.co/edit/8fNus4oiqEFmMXNjq6D1?p=preview" rel="nofollow">PLUNKER</a></p>