S3 authorisation

Adding the authorisation of Amazon users. The implementation closely follows https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html.

The goal is to have simple calls to an authorisation object in the request handlers that then calculate the signature from the message and compare it with the one given in the message headers.

Appropriate errors are send when this fails!

Tests cover the public functions.