Fixing CONTRIBUTING docs

This PR does not just fix the docs (contributing part) but also fixes (I
guess so) some tips about how to create the environment to develop new
features/fix bugs. To accomplish it I changed some that were raising
errors like 'python setup.py test' (nosetests does not run this way).
updated setup.py and requirements.txt was replaced by
requirements_dev.txt.
I also took advantage to fix a type where another
project was referenced instead of Django-organizations.