[PyGreSQL] Idle transactions

[Christoph Zwerschke]

I have just worked a little bit on the pgdb module and fixed the issue 
with the idle transactions raised by Jeff.

I needed to refactor the cursor parameters and at this opportunity made 
all attributes "semi-private" (starting with one underscore) that had 
been "private" (starting with two underscores). I never liked that 
PyGreSQL hides these attributes so strictly. Sometimes it may be helpful 
to access the underlying connection if you know what you're doing. You 
can now also access the source() method for a pgdb connection as had 
been requested recently.

If nobody complains about this change, I'll check it in.

-- Christoph