Getting make to work

Added a 'clean' target but also change the setup and installation in virtualenv. The existing invocation has never worked for me. I think that part of the problem may be that your version of make is using /bin/bash as the shell by default, but I found that I have to set it manually. Also, extracting the requirements through ```$(<requirements.txt)``` is problematic, although I don't know why, so I have added that as a manual step as well.