Log HTTP response headers in debug logs

We've had requests to log the HTTP response headers
in the debug log, as it helps with troubleshooting.

Sample output:

```
2014-01-02 18:24:58,728 - botocore.response - DEBUG - Response Headers:
x-amz-id-2: foobar
x-amz-meta-customfoo: foobar
content-type: text/plain
server: AmazonS3
date: Fri, 03 Jan 2014 02:24:59 GMT
last-modified: Fri, 03 Jan 2014 00:51:32 GMT
accept-ranges: bytes
x-amz-request-id: requestid
content-length: 12
etag: "someetag"
2014-01-02 18:24:58,728 - botocore.response - DEBUG - Response Body:
```