title: Checkbox not showing up in md-select multiple
body: <p>I am trying to enable multi select in my md-select. I have hardcoded <code>multiple</code> to true but md-select isn't showing <code>checkbox</code> besides the value in the drop down list.</p>
  
  <p>I am using AngularJS 1.4.8 and AngularMaterial 1.0.0. </p>
  
  <p>Code:</p>
  
  <pre><code>&lt;div style="background: #f7f7f7" ng-controller="CircleTimeActivityPageController"&gt;
  
              &lt;md-input-container flex="30"&gt;
                       &lt;md-select multiple="true" placeholder="Observations" ng-model="myData"&gt;
                            &lt;md-option ng-value="selectedObservation" ng-repeat="selectedObservation in observations"&gt;
                                      {{ selectedObservation }}&lt;/md-option&gt;
                       &lt;/md-select&gt;
              &lt;/md-input-container&gt;
  
   &lt;/div&gt;
  
  mainApp.controller('CircleTimeActivityPageController', function($scope, $http, $mdToast, $log) {
      $scope.observations = [1.3, 1.2, 1.1];
  });
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/s5dTl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/s5dTl.png" alt=""></a>
  <a href="https://i.stack.imgur.com/vTiam.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vTiam.png" alt="enter image description here"></a></p>
  
  <p>Please excuse my ignorance in AngularJS and AngularMaterial.</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>CODEPEN Example: <a href="https://codepen.io/wingoku/pen/ZLYxNY" rel="nofollow noreferrer">https://codepen.io/wingoku/pen/ZLYxNY</a></p>
  
  <p>Best Regards</p>