Add gitignore for hidden files and site folder

Summary: The repository was not ignoring vim swapping files nor was it
ignoring the desktop hidden services store files which osx creates.
This diff creates an ignore file and ignores both of those file types.
Additionally since we don't want to checkin the actual generated site
this adds an ignore around that folder so it doesn't get checked in
accidentally.

Test Plan: Modify some files within the repository and then do a git
status with vim still open. It should show both .swp files and
.DS_Store files as not staged for commit. Build the site and then run
git status and you should see the _site folder included in the unstaged
commits.  Add the gitignore file and rerun the status and all of the
above listed files should not show up.
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.
Update meta information on index.html

Summary: Prior to this diff the meta information on the index page was
still reflecting the owner of the template rather than Mark. Note the
information in the following screenshot:

https://drive.google.com/file/d/0B6vYbVKRyOPDR080RV9fT0VfckE

After this fix the information looks like this:

https://drive.google.com/file/d/0B6vYbVKRyOPDOXBKUjRSb09sdUU

Test Plan: Generate the site and inspect the meta information at the
top level, twitter, and facebook and ensure that there is no mention of
the owner of the template.
Update 404 page

Summary: Prior to this diff the 404 page was not getting the proper
background image because it was using the pageminimal template. It
looked like this:

https://drive.google.com/file/d/0B6vYbVKRyOPDaVBUalFfMVpwQ1E

After it looks like this:

https://drive.google.com/file/d/0B6vYbVKRyOPDMjh2Yk43U2pwTTg

I looked at updating the pageminimal template to include the proper
information but it looks like it is significantly diverged from the
base home template. As such I thought that the best solution to the
issue was just using the home template, but this might not be the
actual right solution.

This diff also gets rid of the "adorable monkey" as meta information
for facebook and twitter since the actual image does not exist inside
of the assets for the site. Again this might not be the correct fix,
but i don't have access to the assets so it seemed ok. It could always
be pulled out of the git history once the asset is included.

Test Plan: Build the site and then open the 404 page. It should not
have the proper background and header. Inspect the meta information on
the 404 page and see that it no longer references the "adorable monkey"
image but rather uses the default one.