(hopefully) fix opengraph metadata

- Add og namespace using method recommended in http://stackoverflow.com/q/8235687/266309
- Add og:type and og:url, which are required according to http://ogp.spypixel.com/Pogo/checker/
- Use full url for og:image
Merge pull request #12 from waldyrious/patch-1

(hopefully) fix opengraph metadata