Fix TypeError in edit_permissions

Fix a somewhat opaque TypeError failure in edit_permissions when running django-authority under Django 1.5.1, by properly delegating to superclass in creating a form field.

See https://code.djangoproject.com/ticket/20744 for detail on the Django Forms API subtlety that the superclass method works around (removing `request` from `kwargs` before passing them on to field creation).

The error looks like this:

```python
TypeError: __init__() got an unexpected keyword argument 'request'

Stacktrace (most recent call last):

  File "django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "django/contrib/admin/options.py", line 372, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "django/views/decorators/cache.py", line 89, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "django/contrib/admin/sites.py", line 202, in inner
    return view(request, *args, **kwargs)
  File "django/utils/decorators.py", line 25, in _wrapper
    return bound_func(*args, **kwargs)
  File "django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "django/utils/decorators.py", line 21, in bound_func
    return func(self, *args2, **kwargs2)
  File "django/contrib/admin/options.py", line 1205, in changelist_view
    response = self.response_action(request, queryset=cl.get_query_set(request))
  File "django/contrib/admin/options.py", line 960, in response_action
    response = func(self, request, queryset)
  File "authority/admin.py", line 61, in edit_permissions
    FormSet = inline.get_formset(request, obj)
  File "django/contrib/contenttypes/generic.py", line 495, in get_formset
    return generic_inlineformset_factory(self.model, **defaults)
  File "django/contrib/contenttypes/generic.py", line 455, in generic_inlineformset_factory
    fields=fields, exclude=exclude, max_num=max_num)
  File "django/forms/models.py", line 692, in modelformset_factory
    formfield_callback=formfield_callback)
  File "django/forms/models.py", line 424, in modelform_factory
    return type(form)(class_name, (form,), form_class_attrs)
  File "django/forms/models.py", line 212, in __new__
    opts.exclude, opts.widgets, formfield_callback)
  File "django/forms/models.py", line 170, in fields_for_model
    formfield = formfield_callback(f, **kwargs)
  File "authority/admin.py", line 33, in formfield_for_dbfield
    return db_field.formfield(**kwargs)
  File "django/db/models/fields/__init__.py", line 646, in formfield
    return super(CharField, self).formfield(**defaults)
  File "django/db/models/fields/__init__.py", line 499, in formfield
    return form_class(**defaults)
  File "django/forms/fields.py", line 188, in __init__
    super(CharField, self).__init__(*args, **kwargs)
```