Add directive to install the project in development mode

[halfak]

See https://pythonhosted.org/setuptools/setuptools.html#develop-deploy-the-project-source-in-development-mode

[adamwight]

Probably best to add this to the vagrant provisioning, we would just go to the revscoring directory and run python3 setup.py develop. This installs your source repo on the system python path, so you can make changes with the help of the VCS and still run as if it's a normal package.