Automatic Django project finder

This is a first attempt to have pytest-django automatically find manage.py and add its directory to sys.path.

This PR also includes added documentation that describes how this works and how it can be disabled. I have also updated the tutorial to be improved and reflect this change.

This PR fixes #23, #155, #156 

What do you think about this? I would be very happy for feedback, especially on the documentation.

/cc @flub @blueyed @Suor

I would have written this as

    try:
        return possible_values[x]
    except KeyError:
        raise ValueError('...')

but I guess it doesn't matter too much.

Does this really want to raise AssertionError and not ValueError?