update elements and fix some bugs

lib/src/paper-card/paper-card.html

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 <link rel="import" href="../polymer/polymer.html">
 <link rel="import" href="../paper-material/paper-material.html">
 <link rel="import" href="../iron-flex-layout/iron-flex-layout.html">
 
 <!--
 Material Design: <a href="http://www.google.com/design/spec/components/cards.html">Cards</a>
 
 `paper-card` is a container with a drop shadow.
 
 Example:
 
     <paper-card heading="Card Title">
       <div class="card-content">Some content</div>
       <div class="card-actions">
         <paper-button>Some action</paper-button>
       </div>
     </paper-card>
 
+Example - top card image:
+
+    <paper-card heading="Card Title" image="/path/to/image.png">
+      ...
+    </paper-card>
+    
 ### Accessibility
 
 By default, the `aria-label` will be set to the value of the `heading` attribute.
 
 ### Styling
 
 The following custom properties and mixins are available for styling:
 
 Custom property | Description | Default
 ----------------|-------------|----------
@@ skipping 134 matching lines @@
     },
 
     _computeHeadingClass: function(image) {
       var cls = 'title-text';
       if (image)
         cls += ' over-image';
       return cls;
     }
   });
 </script>