[PyGreSQL] datetime.(date|datetime|timedelta)

[Christoph Zwerschke]

> For instance, the connect() function of the pgdb module could take an 
> additional parameter "datetypes" which could be set to None = "str", 
> "datetime" or "mx" = "mx.datetime". 

Need to comment myself again. I noticed that the mx.ODBC module does it 
similarly. They use a connection attribut "datetimeformat" that you can 
set to the following values:

     DATETIME_DATETIMEFORMAT [default] -> mx.DateTime
     TIMEVALUE_DATETIMEFORMAT -> Ticks
     TUPLE_DATETIMEFORMAT -> Python
     STRING_DATETIMEFORMAT -> Raw from the database (string)

I think we should use the same attribute and values, instead of 
inventing anything new. We only need to split DATETIME_DATETIMEFORMAT 
into two subvariants:

      MX_DATETIMEFORMAT [default] -> use mx.DateTime module
      DATETIME_DATETIMEFORMAT -> use Python datetime module

If mx.DateTime is not installed, then DATETIME_DATETIMEFORMAT should be 
the default instead. Or should we better make STRING_DATETIMEFORMAT the 
default for backward compatibility?

-- Christoph