Fixes open graph description

Currently almost every blog post gets "18F builds effective, user-centric
digital services focused on the interaction between government and the
people and businesses it serves" for the open graph description when
shared. (See:
http://www.google.com/webmasters/tools/richsnippets?q=https%3A%2F%2F18f.gsa.gov%2F2014%2F10%2F21%2Fhow-to-run-your-own-3-sprint-agile-workshop%2F,
for example). This happens when pages or posts are missing a description
value in the front-matter.

To address this I've added a second conditional that checks for a post or
page excerpt if the description is missing. This should make open graph
data more descriptive when shared.
fixup! Fixes open graph description
Merge pull request #344 from 18F/open_graph

Fixes open graph description