Add setup.py so this can be installed with pip automatically

Hello,

No need to accept this pull request if you don't want to make changes to a 2 year old example post, but I didn't find any handier way to do PKCS7 padding in Python, so I added a setup.py file (just meta info) so it can be installed with pip in projects with requirements files.

Feel free to suggest any changes there - I put my contact info but only in case you didn't want to be burdened with ongoing support. Happy to change anything you'd like (also added some usage guidelines to the readme).