__value_factory__ should be added when the default is used

[Barry Warsaw]

It should always be possible to get the item factory for an enum through the __value_factory__ attribute (or whatever we end up calling it in the end). It's possible when given explicitly, as is the case for IntEnum:

>>> A = IntEnum('A', 'a b c')
>>> A.__value_factory__
<class 'flufl.enum._enum.IntEnumValue'>

but not if the default is used.

>>> B = Enum('B', 'a b c')
>>> B.__value_factory__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'B' has no attribute '__value_factory__'

When the default is used, we should add this attribute.