Add a setup.py archticture to be able to test properly with tox.
pypy should not be in the list of tested Python versions, by default.
Forgot to include two test files for cart_from_csv a while ago. Adding them now.
Add MANIFEST and .cache to .gitiginore. They are probably generated by tox.
Fix a Python 2 compatibility issue.

I have tested the whole application in Python 3, until now.