add <meta> tags for popular social networks

[luto]

Facebook and twitter parse special <meta> tags on linked pages, to include some of the content right into their interface. While both twitter and facebook specify their own tags, twitter seems to parse both variants. So the minimal useful set of tags - according to twitter - is:

<meta name="twitter:card" content="summary" />
<meta name="twitter:site" content="@twitternick" />
<meta property="og:url" content="http://absolute/url" />
<meta property="og:title" content="A Twitter for My Sister" />
<meta property="og:description" content="In the early days, Twitter grew so quickly." />
<meta property="og:image" content="http://graphics8.nytimes.com/images/2011/12/08/technology/bits-newtwitter/bits-newtwitter-tmagArticle.jpg" />

Most of that information is already present. The twitter nick could maybe be a new conference setting? It can also be omitted for now.