docs: def is_callable(self):
          """The fake can be called.
  
          This is useful for when you stub out a function
          as opposed to a class.  For example::
  
              >>> import fudge
              >>> remove = Fake('os.remove').is_callable()
              >>> remove('some/path')
  
          """
          self._callable = Call(self, call_name=self._name, callable=True)
          return self