* tests/test_dates.py: add tests for date/time typecasting and
adaption.
* psycopg/adapter_mxdatetime.c (mxdatetime_str): add support for
outputting BC dates (which involves switching them to one-based
dates). Also remove broken handling of microseconds.
* psycopg/typecast.c (typecast_parse_date): if the string ends
with "BC" adjust the year value to be a zero-based BC value as
used by mx.DateTime (datetime doesn't support BC dates).
(typecast_parse_time): ignore ' ', 'B' and 'C' in time strings
rather than treating them as part of the seconds part of the time.
