Alter CSRF exemption implementation

The previous implementation of decorating `APIView.dispach` with the
`csrf_exempt` decorator allowed for an easy-to-make mistake where
someone could override the `dispatch` method on a view and inadvertantly
remove the csrf exemption of their api view.

By moving the decoration of the view into the `as_view` logic, it
becomes much more difficult to make this mistake.
Merge pull request #1706 from pipermerriam/piper/use_decorator_mixin_class

Alter CSRF exemption implementation