add support for psycopg1 mxdatetime conversinos

[Mike Bayer]

psycopg1 has some extra requirements for dates. postgres.py has been patched to support PG2DateTime/PG1DateTime... add appropriate conversions to PG1DateTime.

[Former user]

sqlalchemy.diff adds a check for None that the previous patch didn't include.

[Mike Bayer]

i stuck the mxDateTime import into a try/except, committed the patch in changeset:1273