_fold attribute has inverted sense of PEP-495

[pganssle]

See discussion on #57. The _fold attribute should have the inverted sense.

This may need to be a breaking change, as the default value for fold is 0, meaning that, in Python 3.6:

from datetime import datetime, timedelta, timezone
from dateutil.tz import gettz

LA = gettz('US/Pacific')
t = datetime(2014, 11, 2, 1, tzinfo=LA)

t should be interpreted as the first 1AM, not the second 1 AM. This is the opposite of the assumption that has been made in every tzinfo subclass provided by dateutil up until this point.

Given that we've been pretty bad about ambiguous time zone support, hopefully no one is relying on this specific behavior, and the breaking change won't be so bad.

[pganssle]

Implemented in #302.