Add the ability to debug the signature

[cpliakas]

The purpose is to debug the signing in case something goes wrong. This was instrumental in figuring out the "Connection: close" header. It was removed from this library, but it might be worth figuring out how to add it back in.

Code snippet:

import github.com/aws/aws-sdk-go/aws, add the following code:

signer.Logger = aws.NewDefaultLogger()
signer.Debug = aws.LogDebugWithSigning