MSEnum shouldn't required values to be quoted

It's somewhat strange to force a user to quote valid enum values when defining an MSEnum column.

This patch adds a new option, 'quoting' to MSEnum, which defaults to 'auto'. In 'auto' mode, it examines all of the enum values. If they're all quoted in the same way, then the original behaviour is preserved (i.e. the same as quoting='quoted').

Otherwise the values are treated as unquoted values. MSEnum will quote them to generate the schema.