Add sign handler which adds Authorization header

[cbroglie]

This fixes #94

[lsegal]

Thanks for the PR @cbroglie! After some investigation we pulled back on the strategy of generating presigned URL signatures, since this causes issues with other protocols as well. req.Presign() still works, but header signing is now the default mode and both strategies are supported in the signer. It would possibly be worth refactoring some of this out into separate implementations, but the current implementation obsoletes this change, so I will close. I'll take a look at this code if we ever loop around to do a refactor.

[cbroglie]

👍