Pytest mutable object fixture.

Usage:
        def test_something(object):
            # mutate object as needed
Creating reusable "filter_by_tags" function.

Moving the filtering-by-tag logic out of TagsFilter into a reusable function.
Turning off signals dispatch when testing units

Model signals pre_save and post_save will be turned off when running
unit tests because they are interfering with the tests.
Automatically skip slow tests

You can mark a test as slow using:

    @pytest.mark.slow
    def test_something():
        ...

Use the option "--runslow" to tell the test runner to also include slow
tests:
        py.test --runslow ...
Moving neighbors logic into standalone module