Need support for AWS Signature V4

[athomason]

New AWS regions will no longer support the V2 signature protocol that is currently targeted by the library. So far this includes Beijing and Frankfurt for S3. This is the error currently being returned from those regions:

<Error><Code>InvalidRequest</Code><Message>The 
authorization mechanism you have provided is not supported. Please use 
AWS4-HMAC-SHA256.</Message>

To fix this, Signature Version 4 should be supported as well. All existing AWS regions support V4, but third-party implementations might not, so this should probably be optional, defaulting to the V2 behavior. Other libraries have done this by either hardcoding a mapping of region to desired version, or by falling back to V2 if V4 fails.

The change is unfortunately more complicated than just switching from SHA1 to SHA256 since the V4 signature requires the region name to be included. An API change will therefore be necessary to pass this value. #13 may be necessary/useful.