Add Facebook opengraph metadata

Hot on the heels of https://github.com/Hextris/hextris/pull/85, this pull request introduces some basic Facebook opengraph  `<metadata>` tags to make Hextris links look better when shared on Facebook (see screenshots below for before + after comparison). 

It *should* increase engagement and referrals via Facebook :chart_with_upwards_trend:

![screen shot 2014-08-17 at 11 17 33 pm](https://cloud.githubusercontent.com/assets/121322/3948145/6cb51b30-269f-11e4-83e7-721b0a7eda64.png)

Feel free to tweak the verbiage and image. You can read more about [best practices for Facebook opengraph metadata](https://developers.facebook.com/docs/opengraph/howtos/maximizing-distribution-media-content) if you're interested.

Cheers,
  Lee :beers:

/cc @nmoroze