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 also think a ValueError is more appropriate here.

And I also like the `except KeyError`-approach better.