Update default open graph image

Summary: Prior to this diff the default open graph image for the site
was the background image of the application. This led to a story on
Facebook which looked like
https://docs.google.com/file/d/0B6vYbVKRyOPDVG8wR0JKS3ZoVWc , which is
in my opinion less than ideal. This diff adds better support for a
default og image by adding more properties to the dispaly and using
Mark's face as the default image instead of the background. If a better
image is found in the future one would only have to updates the
default_og_image fields inside of the config.yml to use that.

Since Facebook only scrapes the OG information at the time of the
original post this should not have any effect on previous Facebook
shares but should improve any stories created in the future on that
platform.

Test Plan: Build the site locally and inspect the generated html to
see that the additional information is present in the og information.
Load the image url on the actual and make sure it is pointing at the
blog site and see that it is the correct image.
Merge pull request #2 from gmoeck/change-open-graph-image

Update default open graph image