Add open graph metadata for better Facebook sharing

It looks like there's no image shown when you share the `http://firstpr.me` URL on Facebook e.g.
<img src="http://i.imgur.com/05f1nk5.png" width="50%"> 

Here's what it could look like if you introduce some basic [open graph](http://ogp.me/) metadata (feel free to replace the image):
<img src="http://i.imgur.com/JRprHDk.png" width="50%">

You might also consider:

* adding [Twitter summary card] (and appropriate images) metadata 
* adding user-specific open graph metadata and images e.g. [ogre](https://github.com/leereilly/ogre) :trollface:

Submitted for your consideration :smiley_cat: 

Cheers,
 Lee :beers:

**PS:** Thanks for letting me relive the moment when my first PR was merged!