This code passes all the same tests that the existing RFC mime header
parser passes, plus a bunch of additional ones.

There are a couple of commented out tests where there are issues with the
folding.  The folding doesn't normally get invoked for headers parsed from
source, and the cases are marginal anyway (headers with invalid binary data)
so I'm not worried about them, but will fix them after the beta.

There are things that can be done to make this API even more convenient, but I
think this is a solid foundation worth having.  And the parser is a full RFC
parser, so it handles cases that the current parser doesn't.  (There are also
probably cases where it fails when the current parser doesn't, but I haven't
found them yet ;)

Oh, yeah, and there are some really ugly bits in the parser for handling some
'postel' cases that are unfortunately common.

I hope/plan to to eventually refactor a lot of the code in the parser which
should reduce the line count...but there is no escaping the fact that the
error recovery is welter of special cases.

Although '<>' is invalid according to RFC 5322, SMTP uses it for various
things, and it sometimes ends up in email headers.  This patch changes
get_angle_addr to recognize it and just register a Defect instead of raising a
parsing error.

When the new policies are used (and only when the new policies are explicitly
used) headers turn into objects that have attributes based on their parsed
values, and can be set using objects that encapsulate the values, as well as
set directly from unicode strings.  The folding algorithm then takes care of
encoding unicode where needed, and folding according to the highest level
syntactic objects.

With this patch only date and time headers are parsed as anything other than
unstructured, but that is all the helper methods in the existing API handle.
I do plan to add more parsers, and complete the set specified in the RFC
before the package becomes stable.