Add Classbased generic view and Fix "'test_book" table is not found" issue on test

Add classbased generic view called `FilterView` to `django_filters.generic.classbased` module and move functional generic view called `object_filter` to `django_filters.generic.functional` module. These view are linked in `django_filters.views` module so you can use `django_filters.views.object_filter` or `django_filters.views.FilterView`.

Add test for classbased generic view and fix a lot of issue for testing
django_filters. Now you can test django_filters with any Django app
because when testing, django_filters automatically add
`django_filters.tests` to `INSTALLED_APPS` so no more 'test_book table is
not found' issue!

Fix Date depends test issue as well. Some docstring test depends on the
date test executed.