Added more metadata info to posts, including author info (g+)

Added more metadata for search results and social networks sharing.

Now we can include a "description" and "image" to the post frontmatter info, to be used in the post metadata and do a little better seo. The image defaults to the dev-human logo, and the description gets the first 155 characters from the text.

I'm moving the header_image to another pull request since its something else and has nothing to do with the meta data :)