Add and display optional summary for Mesos blog posts

Add a summary field to the blog post YAML file for displaying on various parts of the website including the homepage. An example would be:

"MesosCon Early-Bird Registration Now Open to the first 140 attendees that register for the conference. Join us in Seattle this August!"

Ideally, this summary field would support markup of some kind. Is it possible to include markdown in a YAML file, or would this need to be HTML?

In addition to proposing this field and its usage, we'd want to update existing blog posts (or at least the most-recent 5 posts) to use this field, and update the blog summary on /blog to potentially use this, in addition to the Mesos homepage.