Implement Z=126 special case

[wbolster]

Temporenc now has a special Z=126 value: temporenc/temporenc#2

This should be handled sensibly in temporenc-python.

[wbolster]

This would likely require a special magic object, e.g. temporenc.EXTERNAL_TIMEZONE, and a way for the .datetime() (and friends) method to obtain/use this external tz. Perhaps a tzinfo arg? This should perhaps work sensibly with pytz, which uses tz.localize() and tz.normalize() to convert.