Add S3 SigV4 Presigning

This change fixes an issue with presigning when signature version 4 is used with S3. It also includes a minor fix for determining the signing region name for vhosted classic domains. Example output:

```
https://taylordt-v4test.s3-us-west-2.amazonaws.com/chicago.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Expires=86400&X-Amz-SignedHeaders=host&X-Amz-Signature=48a81a59132d9b93bf81878a36f4aff38ea56107682face528b2a605886a1c33&X-Amz-Date=20140627T181333Z&X-Amz-Credential=AKIAIFM3UXOJ72EAMEYA%2F20140627%2Fus-west-2%2Fs3%2Faws4_request
```

@jamesls, please review.