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

Good catch. I haven't tried this either, but it seems like pytest-cov https://github.com/schlamar/pytest-cov/blob/master/pytest_cov.py#L34 tries to use trylast too (but it seems misspelled?)