Fix tzwin encoding errors

Stop encoding `_dstname` and `_stdname` to bytes with `iso-8859-1`, because we cannot assume that's the
correct codepage. Uses the current solution we use in `tz.py`, which outputs unicode in Python3.
Fixes #92  and http://stackoverflow.com/questions/21296475/python-dateutil-unicode-warning
and https://bugs.launchpad.net/dateutil/+bug/1227221