OpenGraph support

My friend asked me "Why Schema.org, but not Open Graph for social networks?" and I asked myself, why not? So here is the pull request. You can find details [here](http://opengraphprotocol.org/ "The Open Graph protocol") and [here](https://developers.facebook.com/docs/opengraph "Custom Stories | Facebook").
How it looks like on index and article page you can check on [https://terrty.net/](https://terrty.net/ "Paskal's notes") and [https://terrty.net/2014/aff/](https://terrty.net/2014/aff/ "Парашютный спорт, первый шаг: программа обучения AFF - Paskal's notes") with following tools: [1](https://developers.facebook.com/tools/debug "Facebook"), [2](https://dev.twitter.com/docs/cards/validation/validator "Twitter"), [3](https://www.google.com/webmasters/tools/richsnippets "Google+"), [4](https://www.google.com/webmasters/tools/richsnippets "Google"), [5](http://webmaster.yandex.com/microtest.xml "Yandex").
Note: you need description field defined in your article file to fill "og:description" metatag for article.