Suggestion: Add Twitter App Cards

This is similar to https://github.com/Hextris/hextris/pull/106, but with a slight twist. If you enable [Twitter App Cards](https://dev.twitter.com/docs/cards/validation/validator) for http://hextris.github.io/ then tweets containing the main landing page URL (http://hextris.github.io/) will start showing links to download Hextris from the appropriate app stores. E.g. this is what it'd look like on my :iphone::

<img src="https://cloud.githubusercontent.com/assets/121322/4019146/bd37ef66-2a5e-11e4-8382-5ab43473f702.png" width="50%">

Cheers,
  Lee :beers: