Add "I'm a government agency" to the homepage

Now that government agencies are starting to see the light of open government, it would be great to have this site help them.

In the United States, the federal government cannot apply typical open source licenses because it does not hold copyright over its creations. However there are substantive reasons for them to apply CC0. See http://theunitedstates.io/licensing/. And elsewhere in the world where governments can claim copyright, let's encourage them not to use it. Government data is not open (IMO) if any copyright restrictions apply (http://www.opengovdata.org/home/8principles).

The first commit in this pull req adds "I'm a government agency" to the homepage and links to the CC0 license, which I suggested adding a moment ago in #126. Since government agencies are already starting to use CC0, and not the Unlicense, I suggest that the recommendation on this site to be to do what other agencies are already doing, to create the least friction. And it conforms to the guidance at http://theunitedstates.io/licensing/ (which, full disclosure, I am a co-author of).

The second commit recognizes that I broke the beautiful layout of the site in the first commit and offers a way to make it pretty again.

Thanks to The Noun Project for the icon: http://thenounproject.com/noun/federal-government/#icon-No15212