Improve the billing library

[moul]

Complements #118

• Handle end/start of months when computing duration
• Add new entity: servers volumes
• Add new entity: servers public ip address

[QuentinBrosse]

This issue has been automatically closed because it has not had recent activity. Feel free to reopen it if you think that it is necessary.
Note that we are working on a totally new version of the CLI to support all the new coming products.
Thank you for your contributions and stay tuned! 😉