API/ENH: Support/document/test fold argument in Timestamp
Python 3.6 added a `fold` argument in `datetime.datetime` to disambiguate DST transition times that occur twice (in wall time).

https://docs.python.org/3/library/datetime.html#datetime-objects.

Technically `Timestamp` will accept the argument in 3.6, but it's not formally documented or tested.
Additionally since we will still be supporting 3.5 after dropping 2.7, we can add/handle a fold argument directly in the `Timestamp` constructor as well.
