Add ActionMixin
Add ActionMixin test case
Add ActionMixin test Case
Added working tests for get_url_with_string  and get_url_with_view.

Both are meant to fail (because no action mixins used on the model, we expect a 404 error, when using the string, and an Exception raised by the default get_action_name defined in ActionMixin, when using the view class.
Fix imports
Added ActionMixin to BaseModelViewMixins
Add docstring for test_get_url_with_view TestView
Add ActionMixin to BaseModelViewMixins