Follow the package structure

This repository doest follow the package structure (cf other django ldp package). In order to package this properly and publish to pipy, we need to :

• Nest the code into a djangoldp-invoice directory
• Add a setup.py
• Ideally use singular (ie djangoldp invoice without "s")
• Ideally use Customer instead of Client to avoid confusion (https://git.happy-dev.fr/startinblox/djangoldp-packages/djangoldp-project/blob/master/djangoldp_project/models.py#L7)

If you need help, we can organize a meet up.