Implemented AWS v4 signer

This replace the AWS v2 signer by a version that does v4 signing. I've removed the old V2 signer : There is no reason to keep it in, since all regions support V4, and for some regions (eu-central-1, China) it is mandatory.

For more info:
http://docs.aws.amazon.com/general/latest/gr/signing_aws_api_requests.html
http://docs.aws.amazon.com/general/latest/gr/signature-version-2.html

Fixes #3963
also here please include license headers.