Replace the W3DTF date parser

The old code was licensed under the Python license. I believe that
the feedparser codebase is now entirely free of licensing issues.
Bonus: the new W3DTF parser now has 100% unit test coverage.
Reorder the tests, clarify the variable names
Remove the date parsing comparison check

The W3 date time format is a restricted, normalized subset of ISO 8601.
The unit tests that are checking invalid W3DTF formats and values are
actually being parsed by the ISO 8601 date time parser instead of
returning `None` as expected.

This means that the ISO 8601 parser just made it onto my radar since
I don't believe there should be two built-in date/time parsers with
overlapping format support.
Move the psc helper function out of ISO8601 parser
Update the copyrights to include 2013