Moves the site to Jekyll

I moved the site into Jekyll! 

* There are two layout files: `default.html` (for the main site), and `bare.html` (for the linking policy and thank you page). 
* The bare pages now also have the same JS/CSS assets loaded, and same metadata.
* The thank-you page now redirects with JS instead of a `<meta refresh>` tag. The "you will be redirected" copy was moved up next to the main content.
* I rewrote the README and setup instructions.
* I made paths absolute (e.g. `href="/js/18f.min.js"` and not `href="js/18f.min.js"`).
* Some paths changed: `/thank-you/` instead of `/thank-you.html`. Same for linking policy. It seems early enough, and these pages not so important, that this should be fine.
* Fixed what I think is the cause of the mixed content warning we're currently getting at https://18f.gsa.gov.

This is based off of the `devel` branch, so some other un-deployed changes are present, such as removing Lena and the addition of a link to our open source policy.

Now that we're using Our staging environment can't use a path prefix -- and, we want to keep our options of using non-GitHub-Pages-supported Jekyll plugins open, so we need to use a non-GHP staging area. I will need to update our deploy server to set up a staging path and a production path.

Some future development items remain:

* Move site titles and descriptions (that appear in `<meta>` tags now) into the site's `_config.yml`.
* A better "bare" template for our landing pages. It's just so bare. At least, the main page's footer could be migrated over there.
* Move the blog posts from Tumblr into this site. Someone with Tumblr access needs to copy and paste the post data into markdown files and put them in a directory in this repo (or grant me access to do that), to start.
* Once the blog is in the site, we can remove the `jquery.dotdotdot` and `git submodules` dependencies.
* A Jekyll plugin could do the asset compilation instead of gulp, removing a dependency.
* Team photo section should be generated by a `team.yml` data file - this will also let us refactor how we display teammates much, much easier.
* `<rel = "canonical">` links at the top of pages.
* Site's SSL cert needs to be regenerated as SHA-2.
* Site's SSL configuration needs an update.
* Point `staging.18f.us` at the same box, so we can use that as the staging URL instead of `18f.github.io/staging`. (The [staging repo](https://github.com/18f/18f.github.io) can be deleted at that point, I believe.)
* Site needs to be moved out of GovCloud and into 18F's AWS public cloud (@aaronsnow, is this right?)

But I don't think any of these are standing in the way of the merge -- the deploy details can be taken care of post-merge and out-of-repo, and the site here is as good as it is on 18f.gsa.gov right now.