Add S3 client options allowing configuration of path-style access.

This change makes the use of path-style access an explicit option for the S3 client. Currently it is possible to force the use of path-style access by using an IP address in the endpoint URL, this change allows users to better control the behaviour.

This pull request is an updated version of https://github.com/aws/aws-sdk-java/pull/33 as per conversation with Jason.