DEV: Add tz kwarg to get_datetime.

Adds a `tz` kwarg to `get_datetime` to make it easier to request the current time in a particular timezone.  The existing semantics are unchanged, and  `get_datetime(tz)` is equivalent to `get_datetime().tz_convert(tz)`.

Was frustrated by the verbosity of asking "is it 10:00AM EST?" while working on a bug today.

Ping @jbredeche and @twiecki for API review.