Add view_instance helper method

* Simplify api for setting request, args and kwargs on a
  class-based-view instance
Use object() instead of Mock()

* Python 2 does not include unittest.mock
Use cls.view instead of cls.__dict__['view']

* view_instance requires a class-based view, so we don't need to work
  around python 2 unbound methods
Use django.views.generic.View in test_request

* Use the true View.__init__ implementation instead of a fake
Add `view_instance` to README
Merge pull request #38 from incuna/view_instance

Add view_instance helper method