Add incomparable value test.
Change changed_fields to tuple.
Fix reversed conditional.

fields should be added to update_fields if they currently exist, not if
they never didn't exist.
Update pytz for Travis.
Make test.py use 2to3 build under Python 3.

This is such a hack.
Just use six.

You all disgust me.
Improve test coverage.
Merge branch 'release/1.0.0' into develop

* release/1.0.0:
  Improve test coverage.
  Just use six.
  Make test.py use 2to3 build under Python 3.
  Update pytz for Travis.
  Fix reversed conditional.
  Change changed_fields to tuple.
  Add incomparable value test.