Add more rules for US govt. licenses

[pombredanne]

See https://resources.data.gov/open-licenses/ ... for instance:

U.S. Government Works

Data and content created by government employees within the scope of their employment are not subject to domestic copyright protection under 17 U.S.C. § 105. Government works are by default in the U.S. Public Domain. If no other open license applies, the following URL should be used for the license field:

http://www.usa.gov/publicdomain/label/1.0/

See also data we may want to recognize and parse: https://resources.data.gov/resources/dcat-us/