Change license to CC0

Prompted by licensing talk during a GitHub for Government webinar I attended today, I wanted to open this Pull Request to discuss potentially changing our license to [CC0](http://creativecommons.org/publicdomain/zero/1.0/) (outside of the United States, where it is automatically in the public domain).

An extensive (but still ongoing) discussion about what license is most appropriate for government open source software is happening at the White House's [Project Open Data](https://github.com/project-open-data/project-open-data.github.io/pull/135#issuecomment-23299819). As of right now, a consensus seems to be forming around using CC0, as it is provides a bit more legal coverage than the [Unlicense](http://unlicense.org/). That is what this PR implements (for now).

At the suggestion of [this opendata.stackexchange.com answer](http://opendata.stackexchange.com/a/1050), the language is borrowed from HHS's [ckanext-datajson project](https://github.com/HHS/ckanext-datajson#credit--copying). It seems simple and clear, and doesn't inundate this file with the full text of the CC0 license. (I do think it might be worth putting in the human-readable summary of CC0, though.)

The `CONTRIBUTING.md` document may warrant a few tweaks, if this change is accepted, as well.

Thoughts?

@cndreisbach @marcesher @contolini @Burton