Invenio-s3

- Fix number of parts calculations.
- Removed Python 2.7 support.

- Allow for dynamic part size for multipart uploads.
- Adds new configuration variables to define default part size and maximum
number of parts.

- Fixes typos on configuration variables and cached properties.
- Adds AWS region name and signature version to configuration.

- New configuration variable for URL expiration.
- Enhances file serving.
- Unpins Boto3 library.
- Fixes test suit configuration.

- Initial public release.