Numerbay

- added exponential backoff retry for requests

- added warning for encrypted upload when no sale available

- added install dependency for nacl

- added support for per-order artifact upload and encryption
- added support for per-order artifact download and decryption
- added support for per-order direct Numerai submission

- added `get_my_sales`
- added error message for resolving artifact_id in `download_artifact` when no active artifact exists
- set pypi GitHub workflow to run only on tagged commits
- trimmed docs to API reference only

- initial code release