Updated date/time parsing

I've committed changes to tighten up the date/time parsing, while
enabling more edge cases which should have worked but didn't. The
changes have the following effects:

1) date fields like "2001-12-29" need the same delimiter between all
fields. Previously, any of "-", "/", or "." might have been accepted;
now the second delimiter must match the first.

2) Julian day actually works for both input (with a leading "J") and
output (using EXTRACT('julian' from timestamp)).

3) the ISO-8601 identifier for time fields ("T") now works for all cases
I can think of to test. So '20011227T040506.789' and other variants now
are recognized.

4) more cases of time zones are interpreted correctly. Previously, some
cases were sensitive to leading spaces or lack thereof.

5) regression tests still pass, and I have some additional regression
test cases to add after the upcoming 7.2 release (and those pass too).
                   - Thomas