Update to match Polymer 0.12.0

Updated all that needed to be from styling to table iterations to match Polymer 0.12.0.
I think, thoses lines shoud be added manually in a new step in user story 1.

Also in user story 1, the code example for `game_store_codelab.html` is outdated:

```HTML
<head>
  <!-- import the click-counter -->
  <link rel="import" href="clickcounter.html">
  <script type="application/dart">export 'package:polymer/init.dart';</script>
  <script src="packages/browser/dart.js"></script>
</head>

<body>   
  <click-counter count="5"></click-counter>
</body>
```