Use tz.zone for coversion to db representation

Doesn't actually change functionality, just a bit cleaner, since for
db->python we're relying on pytz apis, so probably good to do the same
for python->db.
Add and __all__ designator to top-level
Change self._choices to [<str>, <str>]

Gets field deconstruction working without sacraficing validation.
Fix tests for django 1.6