add overrideable columns to a table with autoload=True

[Mike Bayer]

users want to be able to specify datatypes to columns that were reflected, i.e.:

t = Table('mytable', autoload=True,
   Column('col3', MyType)
)

where 'col3' overrides that column in the table as it was reflected. seems reasonable (and useful) !

[Mike Bayer]

fixed in changeset:904