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 don't know, I just did some copy-paste from the pytest source code I'm afraid, and thought there was a good reason for that I did not bother exploring further :)

https://bitbucket.org/hpk42/pytest/src/aed87f841a762133588d09f17f2ceb050d726569/_pytest/config.py?at=default#cl-846

Slightly related, I also noticed a strange behaviour in pytest's getcfg when I copy-paste stole it. I opened an issue to track it:
https://bitbucket.org/hpk42/pytest/issue/574/getcfgs-parsing-of-command-line-arguments