[SQLObject] some changes to SQL for postgres

Ian, 

just ran a quick test for the check constraint that I added for
EnumCol for postgres and it seems to work.

all changes that I made are in the attached diff and listed in the
changes file.

(It seems that you changed two lines in Col.py between two of my
checkouts from CVS--but you'll recognize those, I'm sure)

let me know how these changes look

--b


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