Add support for SecurityToken (IAM/STS) in V4 request signing

This commit adds support for using AWS credentials originating from STS in V4-signed calls, by adding the Security Token to the request headers.  Usually these credentials come from IAM roles.

No new tests are attached, since testing this would require a little bit of refactoring of the way aws/sign_test.go obtains its aws.Auth object (aws.Auth.token is private).  If the maintainers are interested, I can go ahead and submit those changes as well.