[SQLObject] Patch for Postgres _fromDatabase support

Here's a first try at adding _fromDatabase support for the Postgres
connection:

http://members.cox.net/davecook/pythonstuff/PG_fromDatabase.patch

One thing that makes me a little nervous is the use of eval to get the
default from the string representation returned by Postgres.  I think it's
safe in this context, but I'm still wary of eval.

I also added guessClass as a method to the class.  I left datetime in there
for now.  Postgres doesn't have a datetime type AFAIK (but does have date
and time).

Dave Cook