PostgreSQL support for MiddleKit

Hi,

attached is a patch against MiddleKit to provide preliminary support
for PostgreSQL.  We are using the patch locally, it seems to work
just fine.  Its a bit unpolished but, well,... here it is :-)

Known issues:
    - sample data not supported
    - requires a patch against the PostgreSQL python bindings
      (only 7.2+ supported)

The reason that a patch against the PostgreSQL python bindings is required
is that MiddleKit makes uses of a database-assigned ID for newly
inserted database objects.  It retrieves this through a MySQL-specific
server function.  Postgres has an equivalent in the 'C' glue for its
binding but this equivalent is not exposed by the python-part of the driver.
(its called 'lastoid').

The second reason is that we put mxDateTime support into the python
part of the Postgres driver so that some assumptions MiddleKit makes
about the type of the returned data of datetime columns hold.  I suppose
there is a way to do that little bit of processing in the MiddleKit patch
directly and if anyone can point me in the right direction there, I would
truly appreciate it.

any and all feedback very welcome!

Ingo