Add Open Graph metadata

This adds an icon and possibly more when sharing a page on Facebook
https://developers.facebook.com/docs/opengraph/using-objects/
https://developers.facebook.com/docs/opengraph/creating-object-types/#properties
Gebruik eigen logo voor Open Graph en niet die van de wiki
Better logo for FB Open Graph
Merge pull request #24 from aykevl93/ayke-opengraph

Add Open Graph metadata