Support timestamp without tz

This just lets Go's time package do what it is going to by default
when one is left without offset information.  Hope it's a behavior
people want.

The main change here is pushing down the column type into the parsing
function, as Go's time parsing can't deal with data that only has the
fractional-hour timezone offset when such a fraction exists
(e.g. "-07" for "-07:00", but "-7:30" necessarily retaining its
precision).  This old hack could not be applied unconditionally (it is
wrong when the timestamp has no time zone offset suffix), so this
information lets a correct decision be made.

Clearly, this approach also can't handle the Postgres datestyle
parameter, but that's a fight to pick some other time.

Signed-off-by: Daniel Farina <daniel@fdr.io>