Add Twitter cards

I noticed that there's no additional metadata shown in tweets mentioning the 2048 website e.g.

![screen shot 2014-11-09 at 1 29 45 pm](https://cloud.githubusercontent.com/assets/121322/4969066/6d0fcbf8-6857-11e4-89c0-ab4be83af20a.png)

Merging the attached metadata and [enabling Twitter App cards for your site](https://cards-dev.twitter.com/validator) will make tweets mentioning `gabrielecirulli.github.io/2048/` look like this (and link directly to the website or app depending on the viewer's platform):

![screen shot 2014-11-09 at 1 22 45 pm](https://cloud.githubusercontent.com/assets/121322/4969050/c26115f4-6856-11e4-93a2-a0be5f7831e2.png)

:blue_heart: :two::zero::four::eight: /cc @gabrielecirulli