Tests

Adding tests for google-hangout-button. @ebidel , can you take a look?
Let's make this tests/index.html and use this boilerplate:

```
<!doctype html>
<!-- Copyright (c) 2014 Google Inc. All rights reserved. -->
<html>
  <head>
    <meta charset="utf-8">
    <title>Runs all tests</title>
    <meta name="viewport" content="width=device-width, minimum-scale=1.0, initial-scale=1.0, user-scalable=yes">
    <script src="../../platform/platform.js"></script>
    <link rel="import" href="tests.html">
  </head>
  <body>
    <div id="mocha"></div>
  </body>
</html>
```