Refactored is_naive from a one-liner in make_fogzap_pickle (in which I
had fixed a bug, proving it is too complicated to trust me to
reimplement it correctly all the time) to a method in tzutil.
Doing major rewrite of foglib.Interval class because the underlying
time intervals were more complex to manipulate than I originally
thought and there were subtle bugs in my existing code having to do
with dates in the future and empty intervals. The code doesn't work
yet, but I am committing because I've done lots of work and I don't
want to lose it if something goes wrong.