Deferred models now automatically update only the fields which are
loaded from the db (with .only() or .defer()). In addition, any field
set manually after the load is updated on save.
* Renamed the __unicode__ methods
* Applied the python_2_unicode_compatible decorator
* Removed the StrAndUnicode mix-in that is superseded by
python_2_unicode_compatible
* Kept the __unicode__ methods in classes that specifically
test it under Python 2
Added the ability to update only part of the model's fields in
model.save() by introducing a new kwarg "update_fields". Thanks
to all the numerous reviewers and commenters in the ticket
