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

Pedantic is good and appreciated :)

I intentionally don't want to get into too much specifics here. I thought about reading the file and peek into its content to try to figure out if it is a "real" Django manage.py file too, but I don't think it's worth the hassle. If someone creates a directory called manage.py or a file called manage.py with something completely different, they will just have to disable this and manage the Python paths by themselves.