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.

There's also the "header_image" that can be used to include a header image to the post page. From the original layout, this is how it looks like:

![screenshot from 2014-05-09 14 14 16](https://cloud.githubusercontent.com/assets/293241/2927851/03b7b5fe-d774-11e3-8ce6-86aa60aa7a68.png)