Postgres Adapter: Treat 'time of day' data as a DBStringValue.

In the Postgres adapter, DBDateTimeValue doesn't play nice when there's only a time of
day and not an actual date, which causes display errors.

Adjusting so that TIME_WITHOUT_TIME_ZONE and TIME_WITH_TIME_ZONE (even
though it's deprecated) outputs as a DBStringValue instead which produces the expected
outputs.

Fixes #13.