Added InvariantCulture to NpgsqlCopySerializer

PostgreSQL currently only supports SQL notation for decimal point (which
is the same as InvariantCulture). This causes problems in many non-US cultures. Since Postgres always expects a dot as decimal point, it is good enough to make this fixed in NpgsqlCopySerializer ( and is somewhat faster).

Added static readonly CultureInfo field for use in string.Format() to always use dot as decimal point.