title: how to get value of ion-checkbox
body: <p>I display my checkbox list using this code : </p>
  
  <pre><code> &lt;ion-checkbox ng-repeat="item in listPref"
     ng-model="item.checked" ng-checked="item.checked"&gt;
     {{ item.text }}
   &lt;/ion-checkbox&gt;
  </code></pre>
  
  <p>This is my listPref: </p>
  
  <pre><code> $scope.listPref = [
      {text: 'Cinema'},
      {text: 'Sport'},
      {text: 'It'} ,
      {text: 'Music'},
      {text: 'Theater'},
      {text: 'Conference'}];
  </code></pre>
  
  <p>I try this code to get the text of all selected item</p>
  
  <pre><code> for(var i =0 ; i &lt;$scope.listPref.length ; i++){
  console.log($scope.listPref[i].checked.text); 
   } 
  </code></pre>
  
  <p>I get the message <code>undefined</code> <code>Cannot read property 'text' of undefined
      at Scope.$scope.pref</code> in my console. 
  Can someone help me please.</p>