[py3] Ported django.utils.encoding.

* Renamed smart_unicode to smart_text (but kept the old name under
  Python 2 for backwards compatibility).
* Renamed smart_str to smart_bytes.
* Re-introduced smart_str as an alias for smart_text under Python 3
  and smart_bytes under Python 2 (which is backwards compatible).
  Thus smart_str always returns a str objects.
* Used the new smart_str in a few places where both Python 2 and 3
  want a str.

docs/howto/custom-model-fields.txt

@@ -688,7 +688,7 @@ smoothly:
 2. Put a :meth:`__str__` or :meth:`__unicode__` method on the class you're
    wrapping up as a field. There are a lot of places where the default
    behavior of the field code is to call
-   :func:`~django.utils.encoding.force_unicode` on the value. (In our
+   :func:`~django.utils.encoding.force_text` on the value. (In our
    examples in this document, ``value`` would be a ``Hand`` instance, not a
    ``HandField``). So if your :meth:`__unicode__` method automatically
    converts to the string form of your Python object, you can save yourself