fix unicode and Builder in Python 2

Prevents a `TypeError` when using `Builder.load_string()` with a unicode kv string to create dynamic widgets in Python 2:

````
  File "/home/ryan/git/aeris2/kivy/kivy/factory.py", line 150, in __getattr__
    cls = item['cls'] = type(name, tuple(rootwidgets), {})
TypeError: type() argument 1 must be string, not unicode
````