Relicense project as CC0

I understand that this requires discussion and review, but I thought I'd kick it off with a pull request.

This relicenses the project to be dedicated to the international public domain -- or as close to that ideal as people have figured out how to do, at least. This is a hybrid license -- it uses the [CC0 public domain dedication](https://creativecommons.org/publicdomain/zero/1.0/) to waive copyright internationally, while acknowledging that the work is already in the public domain inside the US.

This is an approach [advocated by a wide swath of civil society](http://theunitedstates.io/licensing/), and is [the default LICENSE file](https://github.com/18F/open-source-policy/blob/master/LICENSE.md) of [18F](https://18f.gsa.gov) (a technology team housed inside the General Services Administration), as laid out by 18F's [open source policy](https://github.com/18F/open-source-policy/blob/master/policy.md).

The public domain is great, and the best place for taxpayer-funded work. I'm not totally sure of how direct the relationship is between this project and the Department of Defense, but since the current license says "(c) 2012 Department of Defense", it seems that from at least a licensing perspective, the work is a government product.