Suggestion: Add Twitter Summary Cards

* **Step 1:** Merge this pull request.

* **Step 2:** Enable [Twitter Summary Cards](https://dev.twitter.com/docs/cards/validation/validator) for http://hextris.github.io/hextris/.

* **Step 3:** Sit back and enjoy the :chart_with_upwards_trend: increased buzz / traffic via Twitter as  `#hextris` tweets start to look like this...

 <img src="https://cloud.githubusercontent.com/assets/121322/4019085/77d9fa38-2a5c-11e4-9219-0d1e9218793c.png" width="50%">

Cheers,
  Lee :beers: