Package information
Basic Document type with a test skeleton
Parsing function for Documents
Reorganize tests
Put Document-field parsers in a 'private' module

Foo.Internal modules seem to be the accepted way of making "private"
methods that can still be unit-tested. I'm not actually unit-testing
these yet, but I will be! I promise!
Move date-format validation into the date parser

Because that's where it belongs!
Unit test for the posted-date parser
Nice error messages for malformed slugs

Geez, testing parsec errors is a pain in the butt. The Eq instance for
Message just checks that they're the same *constructor*; it ignores the
messages. If we actually want to test that the error message relates to
the malformed input we intended to test, we have to do this big dance
with our own equality message. Eesh.