Update all the front matter

Posts now have organized, useful front matter, which includes an image if available. Here's the front matter for [The Encasement Strategy](http://staging.18f.us/2014/09/08/the-encasement-strategy-on-legacy-systems-and-the/):

```yaml
---
layout: post
date: '2014-09-08T12:02:00-04:00'
tumblr_url: http://18fblog.tumblr.com/post/96977653581/the-encasement-strategy-on-legacy-systems-and-the

title: 'The Encasement Strategy: On Legacy Systems and the Importance of APIs'

image: /assets/images/blog/encasement/encasement1.png

description: In 1986 a nuclear reactor known as Chernobyl released harmful radioactivity which spread over much of the western USSR and Europe. The core of this reactor remains a glowing, ineradicable mass of deadly radioactive lava in the middle of a large Exclusion Zone unfit for human habitation.

authors:
- robert
- mhz

tags:
- API
- how we work
---
```

I've also updated each post that has a byline to use the `{% author %}` tag I added where possible. It can still be written freeform, and guest authors don't need to become tags. It also replaces the `<p><em>` tags around the bylines for a `<p class="authors">` tag. Together, it looks like this:

```html
<p class="authors">
  by {% author robert %} with illustrations by {% author mhz %}
</p>
```

I also updated the `{% author %}` tag plugin to choke and raise an exception if it's called with a teammate's slug that doesn't exist. There are a few authors we don't have in the site yet as a teammate (Shashank, Jackie, Alan) and so we'll want to update those posts later with a tag for them when they're on the site.

There were only 4 blog posts that used images. I copied the images they referenced into `/assets/images/blog`, into separate directories per-post, and updated the post to reference them there. I added a brief README into the [`assets/images/blog/`](https://github.com/18F/18f.gsa.gov/tree/staging/assets/images/blog) directory to discourage people from dumping images in it without making a per-post directory first.